Hmm the rumba board has a rep of being fairly easy to setup.
first of all what firmware are you using IE marlin, repetier, or other.
And what part of the firmware are you trying to change.
And last but not least (This may seem like a dumb question ) but are you saving the firmware as you make changes and are you loading the changed firmware on the board? Also you can not have your host software connected to the board when you are loading firmware.
Rumba marlin set up page:
http://reprap.org/wiki/RUMBA
and another tutorial that may help.
http://www.my-home-fab.de/Dokumentatio … A-Board/en
A little more specific inf o will help us help you.
I hope this helps .
tin
Soliddoodle 4 stock w glass bed------Folger Tech Prusa 2020 upgraded to and titan /aero extruder mirror bed
FT5 with titan/ E3D Aero------MP mini select w glass bed
MP Utimate maker pro-W bondtech extruder
Marlin/Repetier Host/ Slic3r and Cura