Replacing heater core is a little tough, but you don't have to remove the whole dash, just remove both sides of the under dash plastics, then remove the center plastics, and move some of the heater tubes out of the way and you will see it. It is a pain, but can be done.
Quiestion is why do you need to replace, is it a leak or just no warm air. Unless it is a leak its not the problem, and rarely it is a clog unless you use only water in your radiator. Just a side note