Yah, there is no plug that I am aware of.
1) I would take the bottom rad hose off & the bottom hose off the overflow bottle. Use a collector for the old antifreeze. Turn on the heater & fire up the engine COLD. Run for it for about a minute & shut it down.
2) Next, replace the the hoses & refill with straight water, run the engine while refilling the bottle for a few minutes. Leave the heater on.
3) Repeat step 1, replace the hoses & now fill with correct antifreeze mixuture. Keep the engine idling with the heater on & keep adding to the bottle until the engine works all the air bubbles out.
Now you will have clean antifreeze. If you want to get real fancy, on the last step use distilled water, its better for your cooling system.