Sub tester9()
Dim rng As Range
Set rng = Cells.SpecialCells(xlConstants, xlTextValues)
For Each cell In rng
cell.Value = StrConv(cell.Value, vbUpperCase)
Next
This worksheet event code will change to Upper case when you exit the cells.
As written works in Columns A through C.
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Target.Column > 3 Then Exit Sub
On Error GoTo ErrHandler
Application.EnableEvents = False
Target.Formula = UCase(Target.Formula)
ErrHandler:
Application.EnableEvents = True
End Sub