How do you replace a heater core in a 1990 Nissan sentra?

Honestly, You probably dont. The heater core is located under and behind the dash. You would have to remove the dash to get to it. But... if you have a small leak, or if it's putting a haze on your windshield, drop a cup of coffee grounds in the radiator fill. It will temporarily seal the hole. But to answer your question. You remove the dash. It's usually on the passenger compartment side, depending on the vehicle. Should be near the Blower Motor. You can't miss it after you get the dash off. Good luck. But I'd try the coffee grounds first.