Unplug the microwave's power cord, which is often located in a cabinet above the microwave.
Open the oven door and remove the screws on the left and right side of the grill, located above the oven cavity.
Remove the grill by pulling it straight off.
Locate the light-housing, located in the center of the cavity behind the grill. Remove the screw and pull the light housing out of the microwave.
Unscrew the old bulb, then screw the new bulb into place.
Place the light housing back into position and secure using the screw that was originally removed.
Replace the grill, securing it with the screws that were originally removed.
Close the microwave door.
Plug the power cable back into the outlet, then open the oven door to ensure that the light turns on.