You actually have to take most of the center part of the dash apart (which is easier than it sounds. Just time-consuming).
And you have to unbolt the steering column from the dash and lower it down to access the heater core. To do this, you have to remove all the dash fascia under the column, and there's also a removeable metal crossbar, part of the dash, that must also be removed. Again, not hard, just time-consuming.
Once you can get at the casing that holds the heater core, you have to disconnect the core and slide it out sideways toward the driver's side of the vehicle, and dropping the steering column is the only way to have enough room.
This job will generate a lot of bolts and screws to keep track of. Stock up on cheap ziplock bags and a Sharpie to mark them with before you get into this.
I've never actually done one of these, so I can't give you step-by-step, but I once looked it up in my Chilton's manual because I thought I had a heater core issue with my '96, and this is what I remember of it.