It depends on the vehicle's make, model, and year. In *some* vehicles, you can access and replace the heater core from underneath. However, in many, especially front-wheel drive vehicles, the heater core is located deep within the dashboard, requiring partial or complete disassembly of the dashboard and potentially the A/C unit to reach it. The A/C unit is often in the way.
There's no single answer. You need to consult a repair manual specific to your vehicle to determine the proper procedure. A quick online search for "[Your Vehicle Year, Make, and Model] heater core replacement" should give you some clues, and potentially videos showing the process.
