Does anyone know how to take apart an A2 Rear sight??? I want to replace the apature

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.

We have forums specifically to handle this question (like Mantenance & Build It Yourself).  This is where the guys who really know this stuff tend to hang out.  Its also where the links to the manuals which tell you (and show you in pictures) how to do this and so much more.