Remove all of the refrigerant from the system and store it in the refrigerant container. There typically is a filling port near the compressor unit where you can drain the refrigerant out of the system into a refrigerant container.
Disconnect the tubing running to the outside heat exchanger and the compressor. You are going to need to switch half of the pipes, so it is generally easier to disconnect everything then reconnect it in the correct order. Also remove the expansion port which is located at the output of the outside heat exchanger.
Screw the output of the expansion port onto the outside heat exchanger's input then connect the tube running to the output of the interior heat exchanger to the input of the expansion port.
Use the piece of tube running between the outside heat exchanger and the compressor to connect the output of the heat exchanger to the input of the compressor.
Connect the output of the compressor to the tube running to the input of the heat exchanger inside.
Switch the thermostats on the heat exchangers. There should also be a frost sensor on the cold heat exchanger that could be integrated into the thermostat.
Refill the system with refrigerant from the container.