It's pretty easy. Unscrew the front cover that covers the rods. Pull out the rod that goes through the front AND rear motor that you want to replace. I'd recommend only pulling it out far enough to clear the rear motor. Make sure you catch the little spacer that will probably fall out so you can put it back in. Unplug the motor and just pull it out.
Plug in the new motor, line it up with the hole, and the hole through the large cam (drink dispenser) and the spacer, and push the rod back in. Test it. Then screw the cover back on. All done.