FFHuber's suggestions are all good. I would add, there are also screw removers commonly called "easy outs" in various sizes. They usually require that you drill into the screw enough for it to grab the screw then back it out. Sometimes a little heat carefully applied will loosen the screw enough to make it easier to remove.

If the threads in the case are in fact stripped, and you tap it out for a larger screw, the load on the head is not balanced and there is the possibility of warping the head. Here is a method that I have used with some success. Drill and tap the hole for a larger screw (two sizes larger if there is enough metal). Using "lock-tite", or similar thread locker, screw in place an ALUMINUM screw. Then cut off the protruding part and carefully dress it flush with the case. Then drill and tap into the resulting plug the original thread size and use the original size screw.