To change a head gasket, first you need to drain the coolant and remove the cylinder head bolts. Once the bolts are removed, lift the cylinder head away from the engine block and remove the old gasket. Clean the mating surfaces thoroughly, replace the gasket, and reattach the cylinder head with new bolts, following the manufacturer's torque specifications. Finally, refill the coolant and test the system for leaks. Note that this is a very complicated repair job and requires significant mechanical knowledge and experience, so it should not be undertaken by amateurs.