Alot of aftermarket head units have the bracket style where you use a "key" to unhook the head unit from the bracket. Once the headunit is unhooked, you can slide it out. The unhook points are on the left and right sides of the headunits. Just slip in the "key" and you'll feel a clicking sound, do the same with the other side and the unit should slide right out.
Btw, the "key" with most manufacturers is just a flat metal peice about 1" x 8". You're basically just using it to push the metal piece on the bracket that is hooked onto the headunit to keep it in place.