How do you replace the thermostat on a 2002 Pontiac Montana van?

To replace the thermostat on a 2002 Pontiac Montana, first ensure the engine is cool, then disconnect the negative battery cable. Remove the upper radiator hose from the thermostat housing and unbolt the housing to access the thermostat. Replace the old thermostat with a new one, ensuring the gasket is properly aligned, then reassemble the housing and hose. Finally, reconnect the battery and refill the coolant as needed.