Your microwave lamp is not accessible from the inside - the outer covers have to be removed - relatively easy if your microwave sits on a counter top but more difficult if it's a built-in unit.
The outer cover is the external skin - it will be fixed on with several screws. Here's a picture of someone dismantling (wrecking) a microwave for a special project but it gives you an idea of what needs to be done...
http://oregonbrickblockrock.com/z.Microwave-008.jpg
Peccavi
February 2009