There are drain and fill bolts on the driver's side (US) of the transaxle. many people remove both to drain the system and then close off the lower bolt and remove the reverse lamp switch and fill it from the top until it hits the bottom of the upper fill plug.

There is a ton of conflicting information on the boards as to what to use to refill it, some of the pros say Mobil 1 5W-30 synthetic and others say 75W-90 gear lube. There is a big spread there.

I will just buy the stupid expensive stuff (1161423) part number from Volvo when I change mine.

...Lee