Unplug your microwave from the outlet.
Turn the microwave around so the back faces you. Remove the screws holding the back cover on the microwave using the Phillips-head screwdriver.
Locate the bulb casing and remove the screw holding the casing to the microwave.
Unscrew the bulb from the casing with your hands.
Screw in a replacement bulb identical to the burnt-out bulb.
Replace the bulb casing. Screw it in place.
Reposition the back cover on the microwave's body. Replace and tighten the four screws you removed in Step 2.