How to set up an Apple Watch if it was used by another person?

The “another” person will have to follow the procedure in this guide published by Apple 👉 What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Once the above has been done by that person, then the Activation Lock will have been removed from the Apple Watch and all data deleted. You can then accept the Apple Watch and follow the instructions in this guide published by Apple 👉 Set up and pair your Apple Watch with iPhone - Apple Support

Axel F.

"What do you do: How do you set up a Apple Watch if it is use by another person"

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Logging Into Another's Device:

Ask this person to remove them seller from it. If you found it laying around somewhere, then be honest about this, handing it in to local thirties, or to the Lost & Found department, of where you found it.