Remove a bottom hose form your radiator and stand well back as you'll get wet.

Run run your car on tick over and the thermostat will let the rest out.
As soon as it does, Switch off
Alternatively, you could loosen a hose, and run the car allowing the coolant to gradually leak out, you can then stand by the cra and keep topping it up with fresh stuff, but this isn;t the quickest - most efficient way tbh.