How do you change a thermostat in a 2002 Toyota Sienna?

To change the thermostat in a 2002 Toyota Sienna, first, ensure the engine is cool and disconnect the negative battery terminal. Drain the coolant from the radiator and remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing, replace the old thermostat with a new one (ensuring the gasket is also replaced), and reassemble everything in the reverse order. Finally, refill the coolant, run the engine to check for leaks, and ensure proper operation.