Open the extra valve to the plunge that isn’t attached to the chiller via a hose and let the water drain out.
If you are inside or the plunge is in a location where you don’t want the water to empty in that location, hook up a garden hose to the plunge and extend the hose where you want the water to be displaced.
The chiller can also act as a pump to help the water drain faster. Hook the garden hose up to the outlet valve on the chiller and drain the plunge.