Drain coolant low enough to remove the heater core hoses without a lot of coolant loss.
Remove the 4 nuts holding the heater fan to the firewall.
Remove the chamber assembly from inside firewall.
Remove outlet with one screw at bottom and tilt upward to disconnect from the core assembly.
Pull the core assembly from the inside.
Remove sheet metal screws to separate core assembly case with the front of case resting on a flat surface (face down).
Lift out the core from the back portion of the core assembly. (or allow the core to remain in the front half of the core assembly and lift the back carefully if the core is lose enough to stay with the front section). Note the orientation of the air duct that is in the rear portion of the core assembly, it will be loose and needs to be properly oriented on reassembly (no fastners-it is held by pressure on reassembly).
Repair or replace the heater core as needed.
Re-assembly is the reverse of the removal. You need to be sure the internal baffle that is laying loose in the front section is properly spaced from the front section by the 4 rubber spacers and that it fits the core properly when you reassemble the core assembly.
Remember to refill coolant when everything is mounted and the hoses are attached. Note the attachment order of all heater hoses in Fig 33.
I installed a heater in my 50 last year, I did it by myself but a helper would be a lot of help. There is one nut that you have to take off last or at least have all the others very loose to remove it. Your helper will need to hold the heater assy inside while you hold the motor and start the nuts.