How do you change the thermostat on a mercury topaz?

To change the thermostat on a Mercury Topaz, first ensure the engine is cool, then disconnect the negative battery cable. Drain the coolant and remove the thermostat housing, which is typically held in place by bolts. Take out the old thermostat, clean the mating surfaces, and install the new thermostat along with a new gasket. Finally, reassemble the housing, refill the coolant, and reconnect the battery.