Disconnect the oven's power supply by unplugging the power cord.
Remove the screws holding the top grille in place and set them to one side. Lift the grille off.
Loosen and remove the screws over the light bulb's housing. The housing is in the middle of the microwave above the door.
Swap the bulb with a matching replacement. Take the burned out bulb with you when you set out to buy a replacement if you think you will need help finding the correct bulb.
Screw the light cover and the top grille back in place with the new bulb installed. Reconnect the microwave's power.