There is a drain plug on the bottom that you can attach a hose to drain it. You just crack it open and in theory you drain it. Most likely it won't be that easy. I would suggest stopping at the hardware store and purchasing about 5 feet of 3/8 soft tubing. Take off the Safety valve on the top of the boiler and then siphon the water out.
Easy way to siphon is to fill the tube full of water. Cap off one end with your thumb and insert the other into the tank. Quickly lower your end and release water into a bucket or on the ground outside.