It can't... uppercase isn't a format, so conditional formatting cannot apply it.
But you can use conditional formatting to draw attention to text that isn't uppercase.
Formula is =NOT(EXACT(A1,UPPER(A1)))
Or you can use a formula to produce the uppercase of text in other cells.
=UPPER(A1) Then copy and paste values to convert the formulas to the values they produced.
Or you can use Data validation to forbid anyone from entering text that isn't uppercase.
Custom, Formula is =EXACT(A1,UPPER(A1))
Or you can use code to convert text to uppercase.
Code:
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Intersect(Target, Range("A1:A100")) Is Nothing Then
Else
With Target
If UCase(.Text) <> .Text Then
.Value = UCase(.Text)
End If
End With
End If
End Sub