See the useful technical thread index at top of this forum. It's not hard to change the heater core, just time consuming. Basically you have to remove the center console, drop or move the dash, drop the steering column to access the heater core box in behind on the passenger side. The box which holds the core is held in place by some nuts and bolts, some accessed under the hood. Also you can do a search in the forums, there are plenty of threads on this. Give yourself at least 3 hours (most likely 4 to 5) if you're doing it the first time.
First, what year car do you have.. did it come factory with a/c? that will determine how difficult it is. You might end up taking the dash out for something you can do through the glovebox
i just had a thread about me to change my heater core, if you got 87-93 you gotta take the dash apart. anything below that year glove box just needs to be taken apart
i just had a thread about me to change my heater core, if you got 87-93 you gotta take the dash apart. anything below that year glove box just needs to be taken apart
... unless equipped with A/C. If you have an A/C car, the job sucks no matter what. There was no dash dropping in my 86. The dash had to pretty much come out to get the job done. Even then, the HVAC box had to be cut to get the core out (unless I was willing to also disassemble the whole A/C).
If you're still running R-12, your best bet will be to plan to cut the box open.
Enough blathering from me though... knowing the year of your car makes a significant difference.