GE Profile XL 1800

It's a little more complicated than Leo2 suggested but the general concept is correct. *NOTE: Turn off power to the microwave first before servicing it!
At the top of the microwave there are three screws.
Remove the three screws and slowly pull down from the top to get the plastic airvent out of your way.
You will notice a plastic vent with two screws. Remove the forward screw and then pull from the top of the vent to access the rear screw. Now you have access to a metal plate above the fan assembly. Remove the screws at the leading edge to lift up the plate. You don't need to remove it, just lift it up!
Now you have access to the metal box over the lightbulb. Squeeze the box and the box with the light bulb inside come out as one unit. You'll know you have the correct bulb as it is attached to two white wires. Remove the bulb. Should be a halogen 120-130 volt with 35 watt brightness. Remember, halogen bulbs should never be handled with your bare hands. The grease from your fingertips heats up and the bulb burns out prematurely. Always handle these bulbs with rubber or cloth gloves. They are expensive little sucks ($7.79 at Home Depot) so handle them with care!
Good Luck ...

Check inline fuse if open replace it .
Open the top cover w/ screw replace bulb same volt and watt.