How do you change a thermostat on a 2006 S40?

To change the thermostat on a 2006 Volvo S40, first ensure the engine is cool, then disconnect the negative battery cable. Remove the engine cover and drain the coolant. Locate the thermostat housing, typically near the engine block, and detach the hoses and bolts securing the housing. Replace the old thermostat with a new one, reassemble the housing, reconnect the hoses, refill the coolant, and reconnect the battery.