Are you replacing the core only or the whole assembly?
I had to replace mine since mine was leaking when I got my truck but opted to replace the whole assembly just to keep it easy. Plus, a new blower motor couldn't hurt.
Replacing the whole assembly is pretty straightforward.
Disconnect the coolant lines. Undo the plastic fasteners for the side vent boot. A couple of the nuts for the mounting bolts are in the air box so you'll have to remove your intake cap and neck to get to them. Then unbolt the assembly and remove. Your rubber grommets for the cooling lines where they pass through the firewall may be hard/rotted so be prepared to replace them. The whole process is in TM 9-2320-280-20-3, Section 10-87 (page 164 in my PDF).
If you're going further and actually replacing the core in the assembly, I can't help with that.
If I were you, I'd recommend installing some aluminum nutserts instead of bolting it back in place with the original nuts. Using the nutserts will mean you won't have to pull the intake cap, neck, and such to remove the heater assembly in the future. I was glad I installed nutserts when I then decided to go back and upgrade the heater to an MRAP heater.