Its up to you whether you empty the cube first. If you do not, put it on a low baking pan to contain any drips. The advantage to not emptying is that you can then test the fans before final assembly. (The pumps and fan will not run if the reservoir is empty.)

If you decide to empty it, take cube to sink and unscrew cap and empty out water from unit.

You may want to stick your fingers in the two circulating tube holes to open up those valves so that you can drain water from the pump side of the unit.