Take a thin (and I mean *thin*) flat-bladed screwdriver, and carefully (and I mean *CAREFULLY*) insert it between the lamp housing and the body panel at the rear of the lamp. Press in on the screwdriver toward the lamp (e.g. toward the front of the car), which releases the tab back there, and the lamp should pop right out.
I can't emphasize *CAREFULLY* enough; you don't want to gouge your body panel!