How do I change the heater core

Get a copy of the manufacturer's service manual, then read through the pages of steps and drawings before you decide if you want to attempt it. You'll need to discharge the air conditioning system and recharge it later. That should be done by a professional with the proper equipment and safety training. They use gloves, safety glasses, and face shields when working with refrigerant. When recharging the system, it has to be pumped into a very high vacuum for an hour to boil out any hint of moisture. If that is not done, besides intermittent AC operation, the moisture mixed with refrigerant becomes corrosive and will corrode the evaporator next to the heater core, and you'll be doing the job again.