Remove the single 11 mm JIS #000 screw securing the dioptric adjustment knob.

When replacing, make sure you do not tighten the screw too much, as the knob will be locked in position if you do.

Before replacing the knob/screw, make sure the grey plastic mount (screw hole area) is aligned with the outer case. If it is not, it means the lens and the knob are not lined up. With a flat screw driver, give the grey plastic a few rotations, and it should fall in place, and realign properly.