This is a common problem. You can get the replacement gear ($25 plus S+H)at:

http://www.odometergears.com/

The instructions for removing the instrument cluster can be found at Bay 13:

http://www.volvospeed.com/volvo_performance.php

Once the instrument cluster is out, you will need a jeweler's philips screwdriver to remove the various layers until you reach the odometer tripmeter. The whole thing is pretty easy and takes less than 2 hours. Hint: Don't forget to reconnect the small wire from the odometer to the cluster when you reassemble it.

Good Luck