This was part of my 2004 V70 R's 90k service, so it all happened in a professional shop with pro tools. My V70 R's AWD system was used on Volvos from 2003 to 2005 model years, so you may apply the concepts from this tutorial to those models, but things may not be exactly the same, like part numbers. Check with your Volvo indy shop/dealer/mechanic if you're unsure.
Like the Volvo AWD P2 Haldex fluid change procedure, there's not much to it. If you can get your Volvo in the air, it'll be much easier because like that procedure, there's no drain plug, so you need to vacuum out the old fluid.
A Warning
Like all maintenance where you put your car in the air, block the wheels and use redundant support objects. For instance, if you jack the car up and suspend it with jack stands, throw something else under there in case something fails. I'm sad to report that a family friend and father of three girls died when his car fell on him while he was under it performing maintenance. Please take safety seriously.
This is a good topic on safely jacking up your Volvo. The model shown in the photos is an 850 and different from your 2001+ Volvo, but the concepts are the same.
Terminology
Bevel gears are sometimes referred to as an angle gears, or simply, differentials or diff.
Bevel/Angle Gear Fluid
Volvo recommends Volvo part #1161620 "Final Drive Fluid".
Crush Washer
Use this copper crush washer! It's very important to not re-use crush washers, or worse, go without a washer.
Change Rear Bevel Fluid on AWD P2 Volvos
Locate the plug just before the gas tank, it faces exactly to the rear.
It's a long reach.
After you get the plug out, you must suck the old fluid out. There is no drain plug, only one that is above the fluid level.
Here Dave is pushing the new fluid into the bevel gear housing.
Close her up and that's that.
Part #1161620.