Drain the cooling system. Just follow your top radiator hose. Where the hose ends, disconnct it. You'll also have to remove the air cleaner duct and the exhaust crossover pipe. Once all that is off (hose, duct, crossover pipe), then remove the Water Outlet Housing (where the radiator hose disconnected---it's attached to the engine with two bolts). The thermostat is then revealed.
Clean all mating services--installation is reverse of removal.