How do I remove heater core 1964 cadillac deville?

You should definitely get a shop manual for this car. It will prove to be very useful. But anyway, the idea is simple but the labor is excruciating. Start with the blower housing on the inside of the engine compartment. This housing has about 8 bolts around it (maybe more maybe less). Once this is accomplished (some people have had to take off the hood to do this, I used a small flat open ended wrench without removing the hood), then you will be able to undo the rubber boot, vents, ducts, defroster cable and wires under the dash. The rubbed boot will be your enemy, I personally ended up making an incision down the corner and using black caulk to repair it during reinstallation. Once the heater core housing is out, then you can dismantle the box until you get to the heater core. When reinstalling, do NOT put pressure on the heater core nipples, or you will have to redo the whole thing when you find out its leaking.