To turn off an Apple Watch, press and hold the side button (the button below the Digital Crown) until a menu appears. Then, tap the power icon at the top right corner and slide the "Power Off" slider from left to right. Alternatively, you can turn off the Apple Watch through the settings menu by pressing the Digital Crown to open the app list, selecting Settings, then General, scrolling down to the bottom, and tapping "Shutdown." The same power off slider will appear to slide and turn off the watch. Note that you cannot turn off the Apple Watch while it is charging; disconnect it from the charger first if needed.