Heater core

Hello,

You must disconnect the battery before you begin and vacuum down the AC system here are to guides to help get you started and diagrams below to show you how to do the job on your car.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

and

https://www.2carpros.com/articles/replace-heater-core

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

Cheers

You may not need to replace the heater core. One possibility is to make sure there is no air in the system. you can do this by removing the radiator cap when engine si cool, turn the heater temperature all the way to hot and bring the engine to operating temperature.
I am giving you a walk through duiagnostic for your situation. Just use the link below to get to the diagnostic;

https://www.2carpros.com/articles/car-heater-not-working