Fully adjustable rear sight? Look close from above.....there should
be small "notch" where the corresponding "tab" on the rear sight blade will slip up and out. The lower portion of the sight blade, i.e. the part that's inside the rear sight housing, has a point on one side that slips inside the little tiny easy to lose spring. The other end of the sight blade is "sharpened" a bit so the screw "clicks" as it's adjusted.
So, remove the windage screw. Carefully maneuver the sight blade so the "tab" slips up through the "notch" in the housing. Move carefully so you don't let the spring "leave the area".
If it's a dovetailed rear sight blade..... a brass punch and a hammer will do the trick.