The subject line, pretty much sums it up. This is the macro I have, as I
input data I want it to change without executing the macro, (i.e, the purpose
of the question.)

Range("A1:A65536").Select
Selection.Replace What:="39", Replacement:="0", LookAt:=xlWhole, _
SearchOrder:=xlByRows, MatchCase:=True, SearchFormat:=False, _
ReplaceFormat:=False
End Sub