How To Change The Bulb In An Advantium 900 Watt Microwave

You'll need to open the door, and remove the screws that hold the microwave face to the cabinet face.
Next, you'll need to remove the microwave from the enclosure. Remove the metal shroud that encases the microwave, and the bulb may then be replaced. Reversing the process gets you back to where you started.
After the second time of pulling the microwave out, I measured and cut a hole in the sheet metal shroud, larger than the access to the light fixture. I bought a metal electrical box cover, and used sheet metal screws to completely cover the hole. This allows me to slide the microwave out about 6" to access the light, instead of the lengthy process of removing the microwave entirely. If you decide to do this, I recommend that you mark the sheet metal while the shroud is loosely fit on the microwave, and cutting your hole away from the microwave, to avoid getting metal filings into the microwave.
Your bulb is a 120 volt halogen, 35 watts with a GY8.6 base. I bought mine at Lowe's, and their item number is 15797, manufactured for Bright Effects. When installing the new bulb, be careful not to touch the bulb, as the oil on your fingers is enough to degrade the bulb, and it will not last as long as it should. Instead, wrap the bulb in toilet paper or a paper towel, leaving the terminals on the base protruding.
Sorry for the bad news, but glad that I could offer a better alternative. By the way, a technician will do the same thing, without the hole, for about $200.