The website mentioned by the other responder is the best information to remove the instrument cluster. You do have odometer gear failure. There are two types of gears that volvo installed in there odometers. You must determine how many teeth your gear has. Once you dismantle the cluster and note the number of gear teeth, you need to look at odometergears.com. The gear costs around $25 and probably costs around 25 cents to mould, however, it beats the Volvo dealer charge of 500-700 to replace the whole instrument cluster when only the tiny gear is broken. You must disassemble the whole instrument cluster to access the odometer. It is somewhat technical and requires some torx micro screwdrivers. Do a google search for "volvo 850 odometer gear failure" a fellow named Tim has a site which gives instructions on how to disassemble and reassemble the instrument cluster. I did this job last fall and was amazed on how a tiny little gear could cause so much trouble.

Dan