Hex editing has been talked about here in the past as one of the limitations of the current version of TextPad. Users have recommended XVI32.
I found another Hex Editor which might be useful to TextPad users. It's called HxD and is also Freeware.
Hex Editor (as opposed to viewer)
Moderators: AmigoJack, bbadmin, helios, Bob Hansen, MudGuard
Hex Editor (as opposed to viewer)
Running TextPad 5.4 on Windows XP SP3 and on OS X 10.7 under VMWare or Crossover.