Once I pulled the instrument cluster, I worked from the back to get to the odometer. You need a very small phillips screwdriver and there are several layers that need to be removed before you get to the odometer. The most important thing is to keep track of which screws go where.

Here is a link to another site that describes the instrument cluster removal procedure. The 4th picture shows the back of the instrument cluster. If you look closely, you can see the screws that need to be removed.

http://www.seawinds.ws/volvo/instrument_cluster.htm

If your cluster doesn't look similar to this, then I'm not sure what to suggest.

Good Luck