You'll need a long, straight punch to move the roll pin.  The set of 4 punches that Bushmaster sells works perfectly, and you'll probably need the other sizes eventually.

First, get out a sheet and sit in the middle of it.  If you sit on the couch, you'll lose some parts...

The roll pin holds the knob on the end of the screw.  Do NOT hammer the roll pin completely out; just move it far enough to clear the screw.  Then, tilt the gun 90 degrees to the right, so the knob side is down.  Carefully pull the knob off.  Note that there is a ball bearing and spring that will fall out.

Once the knob is off, unscrew the adjustment screw from the opposite side.  Don't lose the spring that's under the aperture.  Now, reverse the process.  It helps to have someone around to hold the gun so you can get the knob back on without losing the BB and spring.

-Troy