Unlike most other cars, this fiat thermostat is integrated in the thermostat housing, so you can remove the two 10mm bolts attaching it to the engine (as described by Jolan) using a small socket set, and swap the whole unit. By the way, in all my years I have never come across such peculiar hose clamps as on this fiat - apparently called "sardine can" clamps. The damned things are actually disposable, and must be replaced after removal, so make sure you have replacements handy (you'll probably need 3 - for both ends of the top hose, and one at the bottom of the radiator if you are draining it). Finally, make sure you check that you have added sufficient coolant when you put it all together - I've just done my son's car, and it seems to breed airlocks - after 2 days it seems to have grudgingly stopped needing more water!
Good luck with the project.
Adrian Avery, February 2009