Read the manual found at the old site...perform a search on "manuals" and you should find it.
If not, here goes the easy bit.
Using a fine pin punch (around 1/16"), remove the cross pin on the windage knob. That is the knob on the right hand side of the rear sight.
Be careful removing the knob as there is a detent ball and spring held captive. Capture these with a magnet.
With the knob off, you can unscrew the windage screw from the left hand side using a standard slot screwdriver. There is a leaf spring under the apeture. Note which way it goes in and also remember how the apeture is aligned. There is a reference notch line cut into the large, close range apeture. This is used to reference the scale on the rear sight base. The apeture flips rear for the small, long range apeture.