Locate the defrost timer switch. The location varies per Amana model, so check your manual if you have access to it. Recent Amana models run power from the evaporator through the defrost mechanism, meaning they're near each other in a box towards the back of the freezer.
Pull off the cover plate for the defrost timer. Two screws hold the timer in place. Unscrew the two screws if you ever need to replace the timer. This enables you to pull it out and unhook the wiring.
Turn the timer switch clockwise until you hear it click once. This shuts the fridge off so it defrosts. Turn it clockwise again and the cooling mechanism reactivates once it clicks again.