The in and out lines should be detached from the heater tank to empty as much water from the unit as possible. Water will still remain in the heating tubes so the entire system should be tipped back to lay the system on the tank leaving the tubes facing upwards for the winter as pictured below. This will drain the tubes and ensure that no water gathers in them over the winter. If water freezes in the tubes, the expansion will fracture the tubes. I usually tie the foot end of the tube legs up to something secure to avoid any wind damage to the tubes and their seals in the water tank.
You must use caution in returning the system to its heating position in the spring. Some water will have remained in the tank over the winter and so you must tip the system upright before sunup or after sunset to avoid cold water entering the hot tubes and fracturing them because of the shock of cold tank water entering the hot tubes.
You must also insure that your system is filled with water during sundown to avoid fracturing the tubes. They will always feel cool to the touch but can reach temperatures of 300 degrees inside and thus cold water entering hot tubes will cause the glass to fracture from the temperature shock.