I replaced one in a 97TJ.
If you have A/C then you will have to decharge the system.
I unbolted the dash and tied it to the roll bar as best I could (pulled the passenger side out away from the heater compartment).There are several screws and bolts that hold the heater core housing onto the firewall (some inside some outside of the firewall). Be paatient!! Remove the vacuum lines for the duct controls, heater hoses, A/C lines, heater motor and the housing will slip out of the vehicle (passenger side).
Once out of the vehicle, you will be able to separate the housing compartment into two halfs (top and bottom) the heater core slips out and in with some cusion material (styrofoam) surrounding the core. Make sure that you clean everything very well. ANY left over anitfreeze will create a smell afterwards, especially when the heater gets hot!!!! I ended up getting some cleaner/freshener from NAPA to spray in the A/C compartment and it still took awhile to get rid of the odor (thought my core was actually leaking again).
Reverse the operation and recharge the A/C.
Like I said, it was not that hard just takes time.