How do you replace the thermostat on a 1999 Isuzu Amigo?

To replace the thermostat on a 1999 Isuzu Amigo, start by ensuring the engine is cool and then disconnect the negative battery cable. Drain the coolant from the radiator and remove the upper radiator hose connected to the thermostat housing. Unbolt the thermostat housing, take out the old thermostat, clean the mating surfaces, and install the new thermostat along with a new gasket. Reassemble by reversing the steps, refill the coolant, and reconnect the battery.