I just repaired my wired MM, which was having a problem with the scroll ball. I opened mine up (voiding the warranty), unscrewed the ball mechanism, popped off the plastic cap that holds the ball and watch in horror as parts started flying. Fortunately, I had a clean work surface and didn't lose anything.
Opening it up required taking off the softer rubber sliding ring, and then popping off the outer ring, which appears to be glued on since there are no clips.
Unscrewing the ball mechanism is easy enough but you need a very small philips screwdriver. I used a small flat screwdriver to pop the mechanism open, but again, take care because the parts may go flying. What I found inside was fuzz covered 2 of the 4 tiny magnetic rollers, and no amount of spray air was going to get it off. I cleaned it off but then had a problem putting it back together since I didn't see how it was originally. What I finally figured out was that the rollers were arranged along the sides in a square, with the magnet ends each fitting in its own corner and the rollers facing out clockwise. The ball then fits and the clip will go on without problem.
Putting the rest back together was easy, except that the outer ring needed to be reglued (unless someone here knows otherwise.)
It's on the computer now, and seems to be working as good as new. But I'll have to say, having worked with the old roller ball mouse that can be easily opened, this is not a very friendly design. Mine was so dirty in 4 months of use that it stopped working, and I wonder how many Apple has to exchange under warranty. I'm sure I'll be doing this again in a few months, unless of course I buy something else instead.