It is an Allen or hex screw. It should be 1/16" (.0625 inches). Once you have loosened the screw, your journey has just begun. The sights are fitted very tightly and are very difficult to move. Usually I find some rust on the bottom of the sight, which adds to the difficulty. I manufacture a tool specifically for this task, but other than a few that went to Brazil, I've had little luck getting them through customs in South America.
If you can support the slide solidly in a bench vise, you may be able to move the sight with a brass punch and hammer. You will probably have to remove the safety levers from the slide. Hope this helps you.
