How do I replace the heater core in my 2002 toyota cellica?

divx, this does not look like fun. The heater core (radiator) is located inside the heater unit. Here are the steps to get to it, but I do suggest you get a service manual for this job ;-) Thanks to Autozone.com:

Have the A/C system evacuated by a EPA-certified technician.

Remove the cooling unit.

Disconnect the suction tube and liquid tube from the cooling unit fitting. Cap all fittings immediately to keep moisture out of the system.

Remove the grommets and drain pipe grommet.

Remove the glove compartment assembly.

Disconnect the wiring from the cooling unit, unbolt and extract the unit.

Drain the engine coolant from the radiator.

Unbolt the heater control valve, label and disconnect the water hoses.

Remove the pipe grommets.

Remove the instrument panel and reinforcement.

Label and disconnect the heater unit wiring harnesses. Remove the retaining nuts around the heater unit and extract the assembly from the dash.

Unscrew the heater air duct and loosen the pipe clamps. Pull the heater core with return pipe out of the unit.

Install the components in the reverse order of removal securing all clamps and screws till snug.

Fill the cooling system, then start and warm the engine, making sure it stays full. Stop the engine and check for leaks and top off the system if necessary.

Have the A/C system charged by a certified automotive technician.

Check for proper heater operation.

Now if you can't get to it and you will have to remove the whole heater unit, you will have to remove the dashboard as well as the steering and airbag. Definitely a service manual job:-) I do hope this helps to get you started.