The thermostat is connected by two wires. The wires are connected with slip on connectors or a wiring harness. Firmly pull the connectors or harness off of the terminals (do not pull on the wire). You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be replaced.
Remove the thermostat, which may be secured by a screw or clip, and replace it with the new part. Reconnect the wires, replace the cover, and plug in the refrigerator. Adjust the defrsot timer to start a defrsot cycle. Check back every five minutes until the defrost cycle ends. Watch for overheating.