A UDF is a User Defined Function.
With Bob's SUMPRODUCT formula you don't need the UDF
But just in case...........
Since you're not familiar with VBA and macros, see David McRitchie's site for
more on "getting started".
http://www.mvps.org/dmcritchie/excel/getstarted.htm
or Ron de De Bruin's site on where to store macros.
http://www.rondebruin.nl/code.htm
In the meantime..........
First...create a backup copy of your original workbook.
To create a General Module, hit ALT + F11 to open the Visual Basic Editor.
Hit CRTL + r to open Project Explorer.
Find your workbook/project and select it.
Right-click and Insert>Module. Paste the UDF in there. Save the
workbook and hit ALT + Q to return to your workbook.
Assuming A1 has 123
In B1 enter =revstr(A1) or revstr(A1) * 1
Gord