Look at your cylinder. You should see four pipes. One off the top (hot draw off) two that run parrell into the side (heating flow and return) and one that enters near bottom, this is the one you want, trace this pipe up and you should see a valve at eye level. this will turn off hot water.
If valve is broken, turn off mains, run cold bath tap and you will drain tank that feds both hot and cold water. Replace valve and turn mains on.