My refridgerator is not staying cold in the...

Question

I think it is the defrost thermostat or the defrost heater from everything I've read. I know that I'll have to get in behind the back wall of the freezer but I can't figure out how to get the walls of the freezer out. It is a Magic Chef Model RB214R and I don't have a manual. I found one on Magic Chefs site but they did a crappy job scanning it in and every other page you can only read half of the page. Any help would be appreciatedThanks,Kelly