I did it recently for my CB350. I stripped the instruments to paint the covers, and while I was and it and provided that I have fully rebuilt the engine and painted the bike, I thought it was fair to reset the speedo because it was something like a new beginning.
Never had done before, but it was not very difficult to figure out. The speedo gears are actuated by a screw, so you can't simply turn the digits to the desired reading. It's necessary to remove the digit units and set the reading manually. To do that, with a screw driver pry open the bracket -see atached picture- and remove the wheels. Be careful, there is a spring in the wheel's axis. Between every pair of wheels there is a small metal bracket whose purpose is to be an "index", a stop for the digit to be in the right position. The instrument face put all those small, tiny two-prong brackets at the same level.
Once you have the wheel numbers apart, it's a matter of turning wheels until you reach the desired reading. The indexing brackets must be all in the same position. It's not difficult but it's a little "tricky". I think I moved them backwards because it was a shorter trip than moving forward but I don't remember well. I set them to 99.999,5 or so to make some room for the first test drives...
![Grin ;D](http://forums.sohc4.net/Smileys/default/grin.gif)
Don't know how other speedos are built but i guess they are pretty much the same...