Close the microwave door, and then unplug the power cord from the wall outlet. The outlet is frequently located inside of the cabinet located directly above the microwave.
Remove the screws that secure the top grille of the microwave, which are located on the top of the grille.
Remove the grill by sliding it to the left and then pulling it away from the microwave. It is sometimes easier to remove the grill if you open the microwave door.
Remove the screw securing the light cover in place, and then remove the light cover.
Depress the tabs securing the light socket, and then remove the light socket from the microwave.
Remove the light bulb by pulling it out of the light socket. Insert the new halogen light bulb into the socket.
Reinstall the light socket, using the tabs to secure it into position. Replace the light cover and use the screws that were removed to secure it.
Reinstall the grille on the top of the front of the microwave. Secure the grille to the microwave using the screws that were removed.
Close the microwave door, if applicable, and then plug the microwave back into the wall outlet.
Disconnect the power to the appliance by unplugging it from the wall outlet, or turning off the breaker in your service panel.
Use a screwdriver to remove the screw on the right-side of the light cover, located on the bottom of the microwave. Push upward on the left portion of the light cover to open the cover.
Unscrew, remove and discard the old light bulb.
Screw the new 40-watt incandescent light bulb into the light socket until tight.
Raise the light cover, and then secure it in place using the screw that was removed.
Restore the microwave's power source.