How to change the heater core

This is a really big job that most mechanics do not want to do. It involves recovering the refrigerant from the AC system, then pumping the system into a vacuum before charging it again. Both steps require very expensive special equipment. Also, refrigerant is extremely dangerous to work with. It can cause frostbite and blindness. Professionals wear gloves, safety glasses, and a face shield.

Here is a link to a guide on replacing the heater core:

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

You may find it is best to have a paper copy of the manufacturer's service manual specific to your model.