Answer:
There is usually a hose or drain connection somewhere near the bottom. If it's a hard-sided, then there will be an access panel, probably near where the power and pump are located.
Make sure that it's shut down completely, and the power disconnected. You don't want to burn out the heater or pumps by draining it while it's trying to heat.
If you cannot find the drain fitting, then disconnect the power and either pump or siphon the water out.
Your instruction manual should have information specific for your model.