How do you change a thermostat on 2003 Pontiac Montana 3.4L?

To change the thermostat on a 2003 Pontiac Montana with a 3.4L engine, start by allowing the engine to cool down completely. Then, drain the coolant from the radiator and remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing to access the thermostat, replace it with a new one (ensuring the gasket is in place), and reassemble everything by reversing the steps. Finally, refill the coolant and check for leaks before starting the engine.