How to power off Apple Watch?

You will have to press the side button until you see the "Slide to turn Off" icon.

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• Turn off your Apple Watch: Press and hold the side button until the sliders appear, tap , then drag the Power Off slider to the right.

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copied from: Restart Apple Watch - Apple Support

Or open the Settings on your watch, tap on "General" and scroll down to the bottom of the screen and tap "Power off"

Press and hold this button (see picture)

There will either be a slide control to power off or a power icon you can tap

Restart Apple Watch - Apple Support