I just wanted to concur with the advice given by MadeInJapan.

But rather than use a butter knife to pry up the speaker grills, you'd probably be safer to use a credit card. The wider "blade" and soft plastic is much less likely to damage the dash pad, and the grills don't take much leverage to pry up.

Once the grill is removed, I have to confess that I'm not sure what you're going to see, as my S70 may be a little different from your 850. On my car, the tweeters are held in place with push-clips that are removed by pushing down on the center to release them. Then the speaker can be pulled up, being careful not to drop the loosened clips into the dash. Installation is a little different in that the center pin in the clip must be lifted up prior to replacing the clips, and then the pin is pushed down until it's flush with the rest of the pin in order to lock the pin in place. See the second diagram on the second page at this URL:

http://www.jimrothe.com/volvo/volvo-dynaudio_instructions.html

If you don't have anything that looks similar in the 850, then just disregard my advice.

--
Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html