Here are the diagrams and instructions below on how to remove the dash and replace the heater core there are many so check out each one. Also if you get stuck please take video or images of the problem and upload them here.
Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.
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.