Here is the procedure for the 850 complete with pictures.

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

I did it myself last summer and consider it to be one of the easiest repairs I have ever done. All you need are several different sizes of screwdrivers. From start to finish was less than 2 hours and most of that time was spent in front of the TV advancing the odometer to match the true mileage. The gear was under $30 with tax and shipping. My dealer wanted $500 to replace the odometer and my indy mechanic said that it would be $150 plus he would need the car for 10 days while he sent the odometer off to be repaired.

DAX
'95 850 Turbo

Check out this link:

http://www.brickboard.com/AWD/index.htm?id=680941

I took the instrument cluster out of my 94 850 turbo last year to replace the burned out bulbs. It would be best to have a manual since you have to remove the complete dash cover to access the instrument cluster. Once you get to it, it just comes out after unsnapping a few clips. The cluster is completely electronic and the only connections are electrical. There is no speedo cable or gears as in older speedometers and odometers. You'll have to either send it to a VDO repair shop or buy a new one. Sorry

I havent done it on the 850 myself but I've done it on other vehicles. I would say intermediate since you need be patient and careful not to scratch/crack any parts and to be organized with screws that you dont have any left over in the end because you missed putting it back in. Good Luck.