PHM Registry Editor:
http://www.phm.lu/Products/PocketPC/RegEdit/
There are also editors that come bundled with explorer programs (e.g., Resco Explorer, Advanced Explorer).
I'm no expert at registry editing, but basically you do a regular backup (e.g., using ActiveSync, Sprite, etc.) then you open up the editor and make your changes. Rest assured that if you really mess up and can't restore from your backup, you can also do a hard reset and start over completely (as though you just got your Axim).