Shut the power off to the microwave by unplugging the electrical cord from the wall outlet. If the microwave is a built-in unit, turn off the circuit breaker that supplies it electricity.
Remove the two screws on the bottom of each side of the cabinet using a Phillips screwdriver.
Unscrew any other screws that are on the top or back of the cabinet.
Slide the cabinet off the microwave.
Remove the old light bulb and replace it with a new bulb. Reassemble the cabinet and turn on the circuit breaker or plug the electrical cord into the wall outlet.