In water or on the hard method
This is pretty much the same for any other water pump.Disconnect from the thru-hull and either pump the pink antifreeze or just blow it out with a dinghy air pump.I use the air pump method because then I don't have to spend money on the pink stuff and there is no fear that the antifreeze will affect the rubber seals of the pump.If the boat is on the hard you could blow air into the seacock from the outside and that would save a disconnect/reconnect of the hoses inside.