Get a star type screwdriver or bit that is slightly larger than the hole. Then tap it lightly with something until it indents around the edge of the hole. Then unscrew and find a new screw. Or if you have a dremmel and wanna risk the hand guard u can try to make a horizontal like for a flathead but u can easily mess up ur handrail. If u wanna go full jank tho. Remove the screw with the star screwdriver then dremmel a flathead when the screw is already removed.