If you are talking about converting to upper case in data entry, try using the After Update event of the textbox:
Private Sub YourTxtBox_AfterUpdate()
Me.ActiveControl = UCase(Me.ActiveControl)
End Sub
If you are refering to viewing previous data that you want to see as upper case then try:
Using a field called "Name" from the table/query as an example:
In the Control Source of the text box put:
=Ucase[Name]
HTH
RDH