From your picture it looks like the motherboard can be lifted off as said above. Once the motherboard is removed you should be able to remove the broken screw with a pair of pliers, or remove the standoff (assuming it has standoffs) and replace with a new standoff. If the case doesn't use standoffs and you can't remove the screw even with the motherboard removed, then just leave it as is, the other 8 screws should sufficiently hold the motherboard in place.