Well you may not be able to do so. It depends to some extent on what other channels you are using for a particular model whether or not that channel might be availble and even then if it is, you may not be able to assign it to the switch you'd like as JR isn't really known for switch assignment ability until you get into the newer, bigger radios.
It would help if you can post your setup and what you want to do with the change so guys can offer some ideas?