How do you replace a thermostat on a 1997 Toyota Avalon?

To replace the thermostat on a 1997 Toyota Avalon, first ensure the engine is cool and disconnect the negative battery cable. Drain the coolant and remove the upper radiator hose from the thermostat housing. Unbolt the thermostat housing, replace the old thermostat with a new one ensuring the gasket is properly seated, then reassemble everything by reversing the steps. Finally, refill the coolant, check for leaks, and reconnect the battery.