SqlConnection myConn = new SqlConnection();

            myConn.ConnectionString = "Data Source=192.168.15.252;Initial Catalog=TestDB;User ID=sa;Password=Pdindia123;";

            myConn.Open();

            string strqry = "UPDATE Registration SET  Password = ' " + Text2.Text + " ',PhoneNo = ' " + Text3.Text + " ', Email = '" + Text4.Text + "' WHERE (UserName = ' " + Text1.Text + " ')";


            Response.Write("UPDATE Registration SET  Password = ' " + Text2.Text + " ',PhoneNo = ' " + Text3.Text + " ', Email = '" + Text4.Text + "' WHERE (UserName = ' " + Text1.Text + " ')");

            Label1.Text = Text3.Text;

            SqlCommand myCom = new SqlCommand(strqry, myConn);

           myCom.ExecuteNonQuery();

            myConn.Close();

no error not updating anything.....

Since you are updating a table based on some condition like UserName = ' " + Text1.Text + " ', the condition may not fail as there may not be a UserName in the Registration table satisfying the condition.

Just copyt the UPDATe statement generated by Response.Write into SQL Management studio and check how many rows are updated.

Do not put extra blanks before and after the field,

string strqry = "UPDATE Registration SET  Password = '" + Text2.Text + "', PhoneNo = '" + Text3.Text + "', Email = '" + Text4.Text + "'  WHERE (UserName ='" + Text1.Text + "')";

Do not form SQL-query using string concatenation. It's a bad practice. Use stored-procedure or parameterized query.

Read MSDN article SQL Injection.