How to change thermostat on 2004 Mazda 6?

To change the thermostat on a 2004 Mazda 6, first, ensure the engine is cool and disconnect the negative battery cable. 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 (making sure the gasket is in place), and reassemble everything by reversing the disassembly steps. Finally, refill the cooling system with coolant and check for leaks.