Before replacing the condensor fan, unplug the refrigerator to avoid an electrical shock hazard.
The fan motor is held in place with screws and possibly a mounting bracket. Remove the bracket and the screws and pull the motor back away from the housing.
The fan motor is connected by two or three wires. Label the wire placement on the motor before disconnecting the wires. The wires are connected to the terminals with slip-on connectors or a plug. Firmly pull the connectors off of the terminals (do not pull on the wire). You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be replaced.
Remove the fan blade from the old motor and install it onto the new motor. Connect the wires to the new motor and install the motor into the bracket or housing. Make certain that all of the screws are tightened securely.
Replace the access cover (if any) and plug in the refrigerator.