Lock iPhone watch
How can I lock my lost Apple Watch
Apple Watch Series 3, watchOS 6
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If your Apple Watch is lost or stolen -
https://support.apple.com/en-us/HT207024
M1 Mac mini, macOS 11.4; Watch, series 5 GPS/cellular, watchOS 7.5; iPad Pro 2nd Gen, iPadOS 14.6; iPad Air 2, iPadOS 14.6; iPhone 12 Pro, iOS 14.6; TV 4th Gen, tvOS 14.6 HomePods, audioOS 14.6 and Airport Express w/ Airplay 2