Turn off the power supply in your home's circuit breaker, or unplug the microwave's power cord from the electric outlet.
Locate the metal door on the bottom of the microwave, and remove the screws from the door to open it. Wait at least 20 to 30 minutes for the microwave's bulb to cool before handling it, as the bulb may be extremely hot.
Pull the door latch down to access the bulb, and turn the bulb counterclockwise to unscrew it from the microwave's bulb socket.
Insert a new replacement bulb into the microwave's empty socket. Turn the new bulb clockwise until it is securely in place in the microwave.
Push the bottom door up to secure it back into place, and tighten the screws in the door to lock the door.
Plug the microwave's power cord into the nearby electric outlet, or reactivate the power on home's circuit breaker.