Try a small flat screwdriver.
Also heating up the screw then cooling it down fast could help to remove it.
Place a screwdriver on the head of the screw and tap it with a hammer.
If the head of the screw is higher than the surface, try rotating the screw with a pair of pliers.
Apply some lubricant and wait 20 minutes and try again.
You could try with a small drill bit, but be very careful.
You might not have used the right size screwdriver at the beginning and that could have damage the screw head. That could make it hard to remove it.