To leave X, you need to use a menu. Unfortunately for beginners, this is different for every window manager, and for most window managers, it can be configured in many ways. If there’s an obvious menu, look for an entry like “Exit” or “Close Window Manager.” If you don’t see a menu, try clicking each of the mouse buttons on the background of the screen. If all else fails, you can forcibly kill the X server by pressing CTRL-ALT-Backspace. Forcibly killing the server destroys any unsaved data in open applications.