How do you change a thermostat in a 1991 B2600I Mazda?

To change the thermostat in a 1991 Mazda B2600I, first, ensure the engine is cool and disconnect the negative battery terminal. Drain the coolant from the radiator, then remove the upper radiator hose and the thermostat housing. Take out the old thermostat, clean the mating surfaces, and install the new thermostat with a new gasket. Reassemble the housing, reconnect the hose, refill the coolant, and reconnect the battery before starting the engine to check for leaks.