Hi there,
I just did what you are asking about a couple weeks ago. The bulbs in the temp control switches are easy to change, the tach/instrument cluster bulbs are a little harder. My car is the same as yours.

I'll try and describe it as best I can.

If you pull the 2 temp control knobs off (both driver AND passenger temp knobs) you'll see a screw behind each of them. Take both of them out. Now, with a very flat screwdriver or something, gently pry out that entire assembly that includes the air flow knob, the A/C button and recirc button. Pry a little around the edges, top-bottom-left-right, and it'll come out.
Once it's out, you'll notice right behind it is a small circuit board with 2 bulbs in it. It comes out by prying the little clips around it and gently pulling it out. The bulbs are in little sockets that twist out.
Now, I had a hard time finding the 2 bulbs but eventually got them from a local Volvo shop, not a dealer. Just put it all back together, gently, the in reverse order.
The instrument cluster bulbs, your tach light, are located behind the instrument cluster. You'll have to remove the dash by following these instructions here:

http://www.volvospeed.com/Dashremove.htm

Once the cluster is up where you can reach the bulbs, just twist the sockets out gently and pull out the bulbs. You can get these at Autozone or Advance.

Take your time, be careful, and it's not that bad. I did all of these plus my shifter and cig lighter bulbs (had to remove the center console) on a nice saturday afternoon...and this was the FIRST time I had ever done it on a 94 850.

Good luck!