When changing the defrost circuit board it is a good idea to also change the defrost temperature sensor. The parts can be identified by providing the heat pump brand and model number. Always use the correct factory parts for this repair.

Make sure the power is disconnected for the outdoor unit as well as the inside air handler. The low voltage to the board is supplied from the transformer in the air handler and damage may result to the control system if control power is left on. Remove the old circuit board leaving the wiring intact. Install new defrost circuit board. Reconnect wires one at a time and reconnect on the same terminals for the new board. If there are any changes in the style of the board pay close attention to terminal markings and read any instructions provided.

The defrost sensor is strapped on a lower copper connection on the outdoor coil near the small refrigerant line that goes towards the air handler. It has 2 wires that connect on 2 terminals of the defrost board. The strap around the copper line has an overlap retaining clip that holds the sensor on the copper. I have added a picture showing overlap retaining clip open. Remove the old sensor and install the new sensor. Disconnect old sensor from defrost board and reconnect new sensor.