On my Spacemaker, access for bulb replacement is on the top of the chassis, which is under the top/side cover. From a diagram I looked at, it appears that yours is similar. If so, you have to:
1. UNPLUG the microwave and then remove the three retaining screws that you'll find on shelf or cabinet above the microwave. Get help; the front of the microwave will be heavy when the screws come loose from the top.
2. Rotate the front of the microwave down a little and lift the rear up about a half-inch. At this point, it is free from the rear mounting plate. Set the microwave on a suitable work surface.
3. Remove the top/side cover piece. To do this, remove all the small screws on the top, the back, and the bottom edges on each side. Then slide cover back about an inch and lift it off.
4. In the middle section of the top of the chassis, you'll then see a cover above the location of the bulb. Lift it up to get access to the bulb.
It's a lame design for an over-the-range unit, but that's the way it is.
Here's a link to a diagram of what it looks like after you get the cover off: http://www.partselect.com/MultiModels.aspx?ModelNum=jvm1640sj&searchTerm=img_00115013i05&RefineSearch=0&imgMfg=GE