how do I replace heater control unit

If the fan (blower motor) only works only in high it might be the blower motor resistor and not the switch. Air is directed to the defroster by one of the mode doors, if it's not working it might be a bad door actuator. You can access the blower motor wiring harness relatively easy through the glove box opening, and check for voltage when switching between the different fan speeds. You already know your fan is working.