Unplug the refrigerator from the wall.
Remove the screws around the edges of the back panel of the refrigerator. Pull off the panel and set it aside.
Run a hair dryer on the freezer coils on the back of the refrigerator until all the frost is melted. Use a towel to mop up the melted ice.
Remove the two screws that hold in place the heater units on the bottom and middle coils.
Pull the thermostat unit off the coils with the heater elements and unplug the wiring harness that attached to the thermostat and the two heater units. Pull out the old heaters, thermostat and wiring.
Place the new heater kit over the middle and bottom coils where you removed the old ones and secure them in place using the screws that you removed earlier.
Plug in the wiring harness to the new unit where you removed the old wiring harness to complete the connection. Place the rear panel on the refrigerator and replace the screws around the edges. Plug in the refrigerator.