On my Optima Pro, there is a single screw about midway on the front site, easily visible. I don't know about the sight on the standard Optima, but I would guess the dovetailed sight insert must be pushed out of its slot to reveal the screw. You'll have to do that very carefully, keeping pressure evenly on the body of the sight. Most any gunshop that mounts scopes should have a sight pushing device to safely remove the sight insert. The main risk in doing this is breaking off the retaining screw.

If you try to remove the insert yourself, keep a firm support on the back side of the sight body as you tap the sight insert out to one side with a small dowel of wood or plastic.