Don’t know passcode to update

Why don't you know the passcode to your own watch?

If you forgot your  Watch passcode -

https://support.apple.com/guide/watch/if-you-forget-your-passcode-apdbebcd1d3b/watchos

But if this is someone else's watch, you can never use it. The Activation Lock can only be removed by the original owner. This prevents lost or stolen watches from being used by others.

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