How do you change-thermostat in 6.6 duramax?

To change the thermostat in a 6.6 Duramax engine, start by draining the coolant and removing the upper radiator hose connected to the thermostat housing. Next, unbolt the thermostat housing and remove the old thermostat, ensuring to clean the mating surfaces. Install the new thermostat, reattach the housing, and then refill the cooling system with the appropriate coolant. Finally, bleed the system to remove any air pockets and check for leaks.