I have something similar, but it is set on the entire for.  The code
runs on the before update.

Here is the code.

Private Sub Form_BeforeUpdate(Cancel As Integer)

'confirm with user that this record is to be modified

Dim updRecord As Byte

  On Error GoTo Form_BeforeUpdate_Error

updRecord = MsgBox("Confirm record change", vbOKCancel, "Record
Modification")

If updRecord = vbCancel Then
   Cancel = True

End If

'Dim priordate As Variant
'Dim prioruser As Variant

   Me!DateModified = Now()
   Me!UserModified = CurrentUser()

  On Error GoTo 0
  Exit Sub

Form_BeforeUpdate_Error:

   MsgBox "Error " & Err.Number & " (" & Err.Description & ") in
procedure Form_BeforeUpdate of VBA Document Form_Customers"
End Sub

Hope this helps.

Put an event procedure on the on change event.

Then put the following code in the event procedure:

Field - is the name of the field on the form.  Change this to the field
name that is on your form.

Private Sub Field_Change()

If MsgBox("do you want to change this ?", vbYesNo) = vbNo Then
   Me!Field.Undo
End If

End Sub

Code that control's AfterUpdate event:

If Me!ControlName.OldValue <> Me!ControlName Then
 If MsgBox("Are You sure", vbYesNo) = vbNo Then
    Me!ControlName = Me!ControlName.OldValue
 End If
End If