//method below are codes for connecting the database
        private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = "Provider = Microsoft.Jet.OLEDB.4.0";
            + "Data source= C:\Database\winbase.mdb";
            return mDB;
        }

When i debug, i have a error saying unrecognized escape sequence. I tried adding @ and \\ but i have more errors.

Write the string as follows:

private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; Data source= C:\Database\winbase.mdb";
            return mDB;
        }
This should work!

or if you would like to have the string written in two (2) rows:

private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; " +
                                   @"Data source= C:\Database\winbase.mdb";
            return mDB;
        }

You get the error because C# considers the backslash character (\) and escape character to use to get a special characters (line return or linefeed). There are two ways to solve this problem, the first being the one Mitja posted, by prefixing the string with the "this is a literal string with no escape sequences" character (@) in front of the string. The second is to escape the escape character (use double \\). So these are equivalent:

mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; Data source=C:\Database\winbase.mdb";
mDB.ConnectionString = "Provider = Microsoft.Jet.OLEDB.4.0; Data source=C:\\Database\\winbase.mdb";

Hi. Thanks for replying.
I tried both ways and I got more errors.

this is my coding.
this is the one edited with the second way.

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;

namespace WineForUs
{
    public class Stocks
    {
        OleDbCommand mDB = new OleDbCommand();
        OleDbCommand cmd;
        public int intQty;

        //method below are codes for connecting the database
        private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; Data source=C:\Database\winbase.mdb";
            return mDB;
        }
        //method below are codes to retrieve Product's quantity on hand
        public Stocks(string strPName)
        {
            DbConnect(mDB);//this command calls DbConnect to activate the connection string
            string strSQL = "Select QtyOnHand From Products Where ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Name", strPName);
            // a try catch is used here to catch database errors
            try
            {
                mDB.Open();
                // the line below stores the QtyOnHand read by ExecuteScalar to oQty object
                object oQty = cmd.ExecuteScalar();
                intQty = (int)oQty;
                mDB.Close();
                return;
            }
            catch (Exception ex)
            {
                mDB.Close(); MessageBox.Show(ex.Message);
            }
        } // closing brace for Stocks(string strPName) method
        // method below are codes to update Product's new quantity after a sale
        public Stocks(string strPName, int intQty)
        {
            DbConnect(mDB);
            String strSQL = "UPDATE Products SET QtyOnHand = @Qty WHERE ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Qty", intQty);
            cmd.Parameters.AddWithValue("@Name", strPName);
            try
            {
                mDB.Open();
                cmd.ExecuteNonQuery();
                mDB.Close();
                MessageBox.Show("Stock quatity updated");
                return;
            }
            catch (OleDbException e)
            {
                mDB.Close(); MessageBox.Show(e.Message);
            }
        } 
    } 

}

Hi. Thanks for replying.
I tried both ways and I got more errors.

this is my coding.
this is the one edited with the second way.

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;

namespace WineForUs
{
    public class Stocks
    {
        OleDbCommand mDB = new OleDbCommand();
        OleDbCommand cmd;
        public int intQty;

        //method below are codes for connecting the database
        private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; Data source=C:\Database\winbase.mdb";
            return mDB;
        }
        //method below are codes to retrieve Product's quantity on hand
        public Stocks(string strPName)
        {
            DbConnect(mDB);//this command calls DbConnect to activate the connection string
            string strSQL = "Select QtyOnHand From Products Where ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Name", strPName);
            // a try catch is used here to catch database errors
            try
            {
                mDB.Open();
                // the line below stores the QtyOnHand read by ExecuteScalar to oQty object
                object oQty = cmd.ExecuteScalar();
                intQty = (int)oQty;
                mDB.Close();
                return;
            }
            catch (Exception ex)
            {
                mDB.Close(); MessageBox.Show(ex.Message);
            }
        } // closing brace for Stocks(string strPName) method
        // method below are codes to update Product's new quantity after a sale
        public Stocks(string strPName, int intQty)
        {
            DbConnect(mDB);
            String strSQL = "UPDATE Products SET QtyOnHand = @Qty WHERE ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Qty", intQty);
            cmd.Parameters.AddWithValue("@Name", strPName);
            try
            {
                mDB.Open();
                cmd.ExecuteNonQuery();
                mDB.Close();
                MessageBox.Show("Stock quatity updated");
                return;
            }
            catch (OleDbException e)
            {
                mDB.Close(); MessageBox.Show(e.Message);
            }
        } 
    } 

}

Which lines give you errors, and what errors are they? You need to give us as much information as possible.

okay. this is my codes.

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;

namespace WineForUs
{
    public class Stocks
    {
        OleDbCommand mDB = new OleDbCommand();
        OleDbCommand cmd;
        public int intQty;

        //method below are codes for connecting the database
        private static OleDbCommand DbConnect(OleDbCommand mDB)
        {
            mDB.ConnectionString = @"Provider = Microsoft.Jet.OLEDB.4.0; Data source=C:\Database\winbase.mdb";
            return mDB;
        }
        //method below are codes to retrieve Product's quantity on hand
        public Stocks(string strPName)
        {
            DbConnect(mDB);//this command calls DbConnect to activate the connection string
            string strSQL = "Select QtyOnHand From Products Where ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Name", strPName);
            // a try catch is used here to catch database errors
            try
            {
                mDB.Open();
                // the line below stores the QtyOnHand read by ExecuteScalar to oQty object
                object oQty = cmd.ExecuteScalar();
                intQty = (int)oQty;
                mDB.Close();
                return;
            }
            catch (Exception ex)
            {
                mDB.Close(); MessageBox.Show(ex.Message);
            }
        } // closing brace for Stocks(string strPName) method
        // method below are codes to update Product's new quantity after a sale
        public Stocks(string strPName, int intQty)
        {
            DbConnect(mDB);
            String strSQL = "UPDATE Products SET QtyOnHand = @Qty WHERE ProductName = @Name";
            cmd = new OleDbCommand(strSQL, mDB);
            cmd.Parameters.AddWithValue("@Qty", intQty);
            cmd.Parameters.AddWithValue("@Name", strPName);
            try
            {
                mDB.Open();
                cmd.ExecuteNonQuery();
                mDB.Close();
                MessageBox.Show("Stock quatity updated");
                return;
            }
            catch (OleDbException e)
            {
                mDB.Close(); MessageBox.Show(e.Message);
            }
        } //closing brace for stocks(string strPName, int intQty) method
    } // closing brace for class

}// closing brace for namespace

there are 5 errors.
1. 'System.Data.OleDb.OleDbCommand' does not contain a definition for 'Close' and no extension method 'Close' accepting a first argument of type 'System.Data.OleDb.OleDbCommand' cound be found.
Line 35 Column 21, Line 40 Column 21, Line 55 Column 21, Line 61 Column 21.
2. 'System.Data.OleDb.OleDbCommand' does not contain a definition for 'ConnectionString' and no extension method 'ConnectionString' accepting a first argument of type 'System.Data.OleDb.OleDbCommand' cound be found.
Line 18 Column 17
3. 'System.Data.OleDb.OleDbCommand' does not contain a definition for 'Open' and no extension method 'Open' accepting a first argument of type 'System.Data.OleDb.OleDbCommand' cound be found.
Line 31 Column 21, Line 53 Column 21
4.Argument'2' : cannot convert from 'System.Data.OleDb.OleDbCommand'to 'System.Data.OleDb.OleDbConnection'
Line 26 Column 44, Line 48 Column 44
5. The best overloaded method match for 'System.Data.OleDb.OleDbCommand.OleDbCommand(string,System.Data.OleDb.OleDbConnection)' has some invalid arguments.
Line 26 Column 19, Line 48 Column 19

These errors has nothing to do with the connection string to your database. Your code is a bit weird.
Do you use "using" statement while accessing to db?

like this:

try
{
 using (SqlConnection sqlConn = new SqlConnection( connectionString ) ) 
 {
  //query string and opening the connetion
  sqlConn.Open();
 }
}
catch ( Exception ) 
{
 //catch errors
}
finally
{
 //closing connection
}

Example:

private static void CreateCommand(string queryString, string connectionString)
{
  using (SqlConnection connection = new SqlConnection(connectionString))
  {
    SqlCommand command = new SqlCommand(queryString, connection);
    command.Connection.Open();
    command.ExecuteNonQuery();
  }
}

This is the best approach, I would say.

To ensure that connections are always closed, open the connection inside of a using block, as shown in the following code fragment. Doing so ensures that the connection is automatically closed when the code exits the block.