How do you change a heater core on a 1977 grand prix?

To change the heater core on a 1977 Grand Prix, first, disconnect the battery and drain the coolant from the radiator. Remove the dashboard and the heater box cover to access the heater core, which is typically held in place by screws. Disconnect the heater hoses from the core, replace it with the new unit, and reassemble the dashboard and heater box. Finally, refill the coolant, reconnect the battery, and check for leaks.