I've never had to do one in a volvo, but in my old pontiac i did it a few times. Basically just take the cover off the shifter, and you should be able to see the top of the cable. I'd guess it's a pin kinda thing holding it in there. Once the top end is free, jack up the car and crawl underneath. On the side of the tranny should be the other end of the cable. Same kind of attachment down there. Slide the whole thing out after making sure all the brackets and such are free, and reverse the order for the new one. A new cable shouldn't cost ya more than say, $60 or so i'd guess.

Somebody feel free to correct me on this, as i said, never done it on a volvo...... but that should give you a basic idea.
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John '82 242GLT / '83 244DL / '66 122S (crunched front end, looking for a home)

P.S. I did it in that old firebird about 4 times in a month, because the ground strap from battery to body had broken, and the next easiest path was through the shift cable. That effictively welded the cable to the housing, and fried a bunch of cables before i figured out what was going on!!!!