Follow the steps in the 'Before you sell, give away, or trade in your Apple Watch' section of:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
Then you can setup the watch with your own Apple ID and iPhone:
Set up your Apple Watch - Apple Support
If need be, here are two ways to unpair the watch if you no longer have the paired iPhone:
- See the 'How to erase your Apple Watch if you don't have your iPhone' section of:
Unpair and erase your Apple Watch - Apple Support
- With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.
