The heater core will be removed from the interior of the vehicle. It is located under the dash. When removing it, make sure to cover the area under it to prevent coolant from getting onto the interior components.
To get you started, here is a link that explains in general how it is done. You can use this to help in the process.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.