I would start out but making sure you have the right sized screwdriver and just simply loosen the front screw a few turns. Then try to screw the elevation screw down in a Clock-wise direction. You shouldn't have to remove the front screw unless somebody has bent the whole sight tongue up for some reason.

If the elevation screw is messed up it is held in place by a ring of spring steel that fits into a groove. If the first suggestion I made doesn't work , then I'd suggest you take it to a Pistolsmith and have it done right.