I'm thinking I need to replace the defrost thermostat but I need to make sure that is the problem before I order the part.

We bought this fridge used a few months ago and it worked fine for awhile then the refrigerator box started to warm up. I defrosted the freezer and that seemed to take care of the problem for a few weeks. Today(3/25) I noticed the temp. going up in the ref. again. also, the freezer is turned all the way down but the freezer temp. is about -10 degrees. Does this sound like the defrost stat needs to be replaced and if so, when I order the part can I get step by step instructions on how to replace it?Thanks