this is one of those things that the volvo engineers turned a 10 minute job into a two hour job. before you try and turn the long screw, you need to loosen the bolts that tighten down the screw. You also should loosen the bolts on which the altenator piviots. Be careful when you go to the store to get a replacement belt. Make sure that the counter person uses the metric length marked on the belt (NOT the replacement belt typically shown in their parts books.

more direct to your question.

It is pretty strait forward. If you look at th eALternator, compressor, Water pump you will notice an adjustable bracket with a long screw. You wnat to loosen that and it will allow the alternator, pump etc move freely. If you go to the link below it will give you details on what exactly to do for your car. I did all of my belts in about a hour on a lazy saturday morning.