OK, I found that I had a Gen1 speedo left over from my parts collection, so I took it apart and took some photos. The Gen2 is different in details but identical in concept.
DISCLAIMER: It is a federal felony to roll back odometers. This description is only to show you how to set the mileage of a "new" cluster to match the existing vehicle mileage. Please don't be stupid...
Here is the cluster I have:
When you take it apart (remove clear cover, remove screws holding the front face to the case) and turn it over you see this:
Note the fingers are pointing to a shaft holding 5 (or 6 if you have 6-digit odo) little gears. These gears must be disengaged from the odometer wheels in order for you to turn the odometer wheels to the necessary setting. The gears are on that little pin (shaft) which in this case is held in by a knurl on the shaft near the right finger. Some other odometers have this shaft snapped into place. You need to get that pin loose so you can lift the little white gears off the odometer wheels. I did this by using a slip-joint pliers to push on the pointed end of the shaft while holding the right bracket. Then it looks like this:
Now you can turn the odometer wheels to the correct positions.
It is imperative that you get the little white gears back together correctly. This will take you a few tries. You will notice that the gears have alternating short teeth and long teeth. (Look at the left side of the little white gears in the picture.) The way it works is the short part of the tooth engages every tooth on the odometer wheel to the right of the white gear, and the odometer wheel on the left side of the little white gear only has one tooth slot, so that once per revolution of the odometer wheel to the left of the little white gear it indexes the odometer wheel to the right of the little white gear one set of teeth (one position). You have to get these all back in the right way for it to work...
Here is my "new" Gen 1 cluster:
Hope that helps!