If its like the 80's truck lights (two lights with separate switches, separate from the dome light), you have to grab the front edge by the buttons and pry outward (like going towards the back of the truck), then slip it off the mount.
Hard to explain, and its sorta tough to pop out, but there's a couple of tabs where it meets the ceiling on the edge (same edge as the two buttons) that you're trying to pull the frame away from. Don't worry though, the more you take it out, the easier it will be to remove it!:biggrin2:
EDIT:
Okay, just went to the garage and realized I have it backwards! Ignore the above and do it this way:
Push forward (towards the windshield) here and the opposite side (can't show both hands, since I had to take the photo!):
Then, with your pointer finger, pull this downward on both sides:
It should pop down, you can then slide it rearward over the switches. Here's the inside:
In case you wanna see where the 4 tabs are, here ya go:
And now I need to go clean the cover and vinyl headliner! Good god its filthy in my truck!!!!