How do you change the thermostat on a 2004 Mazda 3?

To change the thermostat on a 2004 Mazda 3, first, ensure the engine is cool and disconnect the negative battery terminal. Drain the coolant from the radiator, then remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing, replace the old thermostat with a new one, and reassemble the housing and hose. Finally, refill the cooling system with coolant and reconnect the battery terminal.