Unplug the electrical cord for the microwave. Place the microwave somewhere you will have plenty of room to work on it.
Remove the screws located at the bottom of each side of the microwave cabinet using a Phillips-head screwdriver. Then remove the screws in the back of the cabinet.
Slide the cabinet off of the main body of the unit.
Locate the light bulb and remove it. Replace the light bulb with a new bulb.
Replace the cabinet and reinsert the screws to the back and bottom to secure it in place. Plug the electrical cord into the outlet.