I want to change the rear sight aperture on an A2 upper and I have the spare part (flip aperture).
Is there anything special to do other than removing the screw opposite the windage knob?
Is the screw pattern forward (or reverse?)?
Any help would be GREATLY appreciated.

ar15.com/content/guides/parts/upperreceiver.html

First you need to push out the roll pin in the knob.  Don't push it all the way out - just enough so you can pull off the knob (about 2/3s of the pin should stick out of the knob).

Watch out for the ball bearing detent * spring as you pull off the knob (it will drop - have a towel underneath the rifle.

Then unscrew the screw normally and install the new aperture (don't drop the spring under the aperture).

I'd advise downloading the -23 and reading chapter 3 as it covers this procedure.  You can download the -23 by clicking on the information button above, look for downloads/manuals, then look for the TM that ends in -23&P.

There is a little roll pin in the elevation adjustment knob that you're going to want to drive out.  Once that  is done the knob can be unscrewed.  There is a spring and ball bearing under it that accounts for the click adjustments.  Normal right hand threading.  there is a flat spring under the aperature as well.  the tricky part is holding it all together when you're putting it back together again.  a third hand helps!

HTH

Jeff

Dang!
You guys are fast!

There are (2) springs and 2 ball bearings.......is only one (1) needed?

ETA
There were damn near 70? clicks to near center when I initially attempted to BZO the sight.
Is this indicative of a NM rear sight?  I don't remember my A2 20" ever having that many clicks?

ok well I check the above diagram....on-site, and only one (ball bearing is shown).

Took out the extra spring and bearing and now the click adjustments aren't so mushy and frequent.....apparently this sight was assembled using (2) of each and replaced both on the initial changing of parts.....but nonetheless,

I knew I could find my answer here.
Thanks all who replied

tagged for the info