Dim sqlupdate as String
Dim myupdateexe as OleDbCommand
Dim sqlupdate = "UPDATE item_table SET Total = Total + " & DropDownList1.SelectedItem.Value & ""
'Do not make the mistake of putting it in 'string' as that indicates it as a string. since you are adding integers together, you cannot use 'integer', it must just be integer. If you are using strings, then add back the 'string'.
myupdateexe = New OleDbCommand(sqlupdate , con)
myupdateexe.ExecuteNonQuery()
 

Dim sqlupdate as String = " UPDATE item_table SET Total = Total + '" & DropDownList1 & "' "

IsNull(Total, 0)

Dim sqlupdateAs String = "Update item_table SET Total=Total+" & DropDownList1.SelectedItem.Value & ""

Dim cmdSelectTotal As OleDbCommand
Dim conString As String ''your connection string
Dim sqlupdate As String
Dim intTotal As String
Dim conn As OleDbConnection
conString = "yourconnectionstringorreferencetoweb.config"
conn = New OleDbConnection( conString )
sqlupdate = "SELECT Total FROM item_table"
'For the above line, you should have some comparison for WHERE
'you're pulling the Total from, like a SessionID or UserID, or as
'long as you retrieve only one record you're fine.
cmdSelectTotal = New OleDbConnection ( sqlupdate, conn )
intTotal = cmdSelectTotal.ExecuteScalar()
if IsNull(intTotal) then
  intTotal = 0
end if
if IsNumeric(DropDownList1.SelectedItem.Value) then
  if intTotal >= 0 and IsNumeric(intTotal) then
    intTotal += DropDownList1.SelectedItem.Value ''or .Text
    sqlupdate = "UPDATE item_table SET Total=" & intTotal & ""
    cmdSelectTotal.ExecuteNonQuery()
  else
    intTotal = DropDownList1.SelectedItem.Value ''or .Text
    sqlupdate = "UPDATE item_table SET Total=" & intTotal & ""
    cmdSelectTotal.ExecuteNonQuery()
  end if
else
  lblError.Visible = True
  'The above line assumes you have some kind of label to represent
  'an error incase it shows, which happens from time to time.
  lblError.Text = "We're sorry for the inconvenience, but the table
  was not updated correctly. Please repeat your steps and try again.
  'OR JUST set the intTotal to a new value, but this depends on what
  'the code is being used for.
end if

Dim updatequery as String = " UPDATE table1 SET table1.columnA = table1.columnA + '" & dropdownlist1.selectedvalue & "' "