The photo isn't clear enough to see the issue.   I'd just drill them out with a left handed drill bit and find new screws.   

Dave

A piloted counterbore of the correct diameter is one method, but an end mill will also work, if you have the machinery and inclination to do the often tricky setup of an odd shaped piece, but even that can lead to some other problems and related damage to the screw and the piece the screw is turned into.  The counterbore or end mill, can grab the screw and because of the tool rotation, unscrew it into sudden deeper contact with the rotating cutter.  Then the real fun begins.  I've found the easiest way and least likely to cause damage to the piece that the screw is screwed into is to determine and mark the correct location for the notch in the screw head, remove the screw, clamp onto it so you can access where you want the notch, and then carefully file the notch with a round needle file.  The needle file will be used to make the cut in line with the length of the screw so no rotational surprises.  Don't attempt to make the cut full size in one shot.  Test fit as you go and you can get a near perfect fit and won't have to do it twice or repair an expensive part.

Hayface