Most smaller shops (and even larger ones, like Sears) will
install a battery for free. Nothing special about this task on
a Volvo (just make sure you have your radio code and a ball-point
pen to reset the clock -- thats all you need).

If you do it yourself I would add (to previous posts):

Wear eye protection (goggles or safety glases) when working
with batteries.

Take a second to study the plastic clamp that holds down the
battery. Remove it then put it right back (with the old battery)
so you see how it works. It can be a bit tricky to replace
if you just remove it without noticing how to re-install.

The battery shelf will hold two size batteries. Stock, the car
came with the smaller one (less weight). Thought I'd mention that
in case the dealer offers you a choice of two (the larger battery
typically costs a bit more).

My personal choice was an Interstate (in the same size as original).