I need to remove a shower frame. It seems that I first need to remove the top horizontal bar. It is, however, screwed from the inside of vertical frames. How can I unscrew them and then screw back? Unscrewing seems hard enough, but even if I do that it seems they once I try to screw them back the first thing they'll do is fall inside the vertical bar.

Upper right corner, seen from the outside:

View from the top opening:

The width of the opening, and thus distance of the screw heads to the wall is around 8 cm.