How do u replace heater core

Open hood and disconnect the negative battery cable remote terminal from the remote battery post.
Remove the HVAC unit housing from vehicle. Refer to: Heating and Air Conditioning, Housing Assembly HVAC, Service and Repair, Heater-A/C Housing Removal and Installation, See: Housing Assembly HVAC Service and Repair Heater-A/C Housing Removal and Installation
With the HVAC unit housing out of the vehicle, remove (2) screws retaining the heater core. Then pull the heater core out of the heater housing.

INSTALLATION
For installation, reverse the above procedures.

Push new heater core into the unit housing.
Fasten heater core to HVAC unit housing with (2) screws.

Hvac unit

Open hood and disconnect the negative battery cable remote terminal from the remote battery post. This must be done to prevent accidental air bag deployment.
Remove instrument panel from vehicle. Refer to: Body and Frame, Interior Trim, Dashboard/Instrument Panel, Service and Repair, Instrument Panel Assembly Replacement, See: Body and Frame Interior Moulding / Trim Dashboard / Instrument Panel Service and Repair Instrument Panel Assembly Replacement
Using an A/C recovery unit, remove all R-134a refrigerant from the A/C system. Refer to: Heating and Air Conditioning, Service and Repair, Refrigerant System Evacuate, See: Service and Repair Refrigerant System Evacuate
Drain the engine cooling system.
Remove air cleaner hose and air distribution duct from the engine.
Remove spring type fasteners to heater hoses at the dash panel and remove hoses from heater core. Plug the heater core inlet and outlet tubes to prevent anti-freeze from spilling on the vehicle interior during removal. If an appropriate plug cannot be found pull back carpet and use caution when removing unit. Keep the heater tubes elevated to prevent spillage of coolant.
Remove one nut at expansion valve retaining both A/C lines to expansion valve. After removing lines cap the expansion valve openings and the A/C hose openings. This will prevent any dirt or moisture from entering the refrigerant system during servicing.

CAUTION : The lubricant used in this air conditioning system absorbs moisture readily (Similar to brake fluid). Do not leave any portion of the system open for extended periods of time.

Remove the three retaining nuts from the studs. These studs project through the dash panel into the engine compartment.
Remove two screws to defrost duct and remove.
Remove two nuts, two screws attaching HVAC housing to dash panel.
Remove four nuts retaining rear seat heat duct and remove.
Remove rear seat heat duct elbow push pin fastener.
Disconnect HVAC harness connector.
Gently pull HVAC unit housing rearward from dash panel, being careful not to spill any coolant into passenger compartment.

INSTALLATION
For installation, reverse the above procedures.