Where is the thermostact located on a 2000 volvo s80

If it's the 5 cylinder DOHC engine then you have to take off the inlet manifold I think, I've done it before but many job's ago!
Basically there is a housing on the front left hand side of the engine with a radiator hose going into it, thermostat's in there.
I would recommend not disconnecting the battery if you have an auto as you (normally should before any work) as the gearbox forget's all it's setting's & you have to re-train it by doing 40 keystrokes sunday driving or it stall's all the time for a few week's.
Hope this help's if not I downloaded the full service manual for my Volvo on a google search, so your's should be on there.

The thermostat is in a metal housing at the end of the radiator hose where it connects to the engine.

The housing is held down by two bolts.
Remove the bolts, remove the thermostat and scrape the gasket and sealant from the housing and the manifold.

When replacing, use Permatex #2 - non-hardening gasket sealant.