If you have all the screws out, just give it a solid jerk. The sealent is holding them down. You can go around the edge with a box cutter or razor blade, but be careful not to push too hard as there is a lip around the entire edge that is made of aluminum and you don't want to break of cut a leak path in the aluminum. Don't cut yourself either. I did this repair a year or so ago, still no leaks.