If this is the notorious allen bolt at the base of the fork slider in the axle holder, loosen it while the forks are still assembled. Better yet, use a racheting strap to compress the fork and then turn the allen bolt. Heat helps too.
One of the best tools for removing old screws is an impact screwdriver. Paid $5.00 for mine and has save much time, money, effort and countless "easy out" screw removers. Trash the old screws and replace them with anti-seize covered allen screws.