Unplug the fridge and remove the shelving, ice tray or dispenser, and any food from the freezer so you have full access to the freezer space.
Search for the access panel inside your freezer. Depending on the specific model, it should be either along the back wall or under the bottom of the freezer. Use a screwdriver to take out any screws and lift the panel off.
Unscrew the evaporator from its position under the panel and either lift it out or move it so you can reach the defrost heater underneath it.
Pull off the slip-on connectors that hold the defrost heater in place, and pull it out.
Slide the new defrost heater into the same space and attach the wire connecters. Screw the evaporator back in place.
Replace the cover of the access panel and reinstall the shelving. Plug the fridge back in and put your food back in the freezer.