Where is the thermostat located and how difficult is it to replace it yourself. on 2002 Volkswagen Passat

Puurchased the car recently has driven fine for over a month. Recently drove it out of town about 160 miles round trip and over heated. I think it's the thermostat. Car start runs fine no knocking, but if I fill with water and drive around the neigborhood it over heats.

Asked by for the 2002 Volkswagen Passat