1. Click on “Steam” in the toolbar at the top left corner of your Steam session.
2. Select “Exit” or “Exit Steam.” Your current session of Steam will close. Alternately, right-click on the Steam icon in the notification tray on your desktop, and select “Exit.” In Windows, the notification tray is located at the bottom right corner of the desktop; in Mac OS X, the notification tray is located at the top right corner of the desktop.
3. Double-click on the Steam icon on your desktop to relaunch Steam. If the Steam shortcut icon is not located on your desktop, search for Steam in the “Start” menu in Windows, or in the Applications folder on Mac OS X.