Some information on this from here:
"1 Unplug the power cord from the wall outlet.
2 Press the door-release button to open the microwave door.
3 Unscrew and remove the screws that hold the vent cover in place on the left side of the microwave interior.
4 Slide the grille to the left. Pull straight out to remove.
5 Remove the single screw above the door near the center of the inside of the microwave oven that secures the bulb holder in place.
6 Lift up on the bulb holder to move it into position where the bulb can be replaced.
7 Unscrew the lightbulb from the bulb holder. Remove the bulb.
8 Screw the new lightbulb into the bulb holder.
9 Push the bulb holder down until it is back in its original position. Replace the screw that holds the bulb holder in place.
10 Replace the grille and the screws that hold it in place." you can also get more info from here Hope this helps, good luck