Get a pot of boiling water, put the wheel in it, put a lid on the pot and let it boil for 15-20 min. Take it out and let it cool (unles you like burned hands) and you should be able to peel the tire back from the rim. If it won't come off, reboil it.

Clean any dried glue off of the rims, then follow these basic instructions for putting the new tires on: http://www.rcnitro.com/rn/articles/tires_1.asp

Just be sure to use rubbing alcohol to clean the bead of the tire before you put it on.. the things are completely covered in mold release agent.