Apple Watch owners will want to unpair their Apple Watch from their iPhone if they plan to sell or give it away. To do this open the Apple Watch app on your iPhone and navigate to My Watch > Apple Watch > Unpair Apple Watch, and tap Unpair [watch name]. This will unpair your Watch from your iPhone and remove it from your iCloud account.
Don't forget, the Apple Watch creates an iCloud / iTunes backup every time it is paired with your iPhone. The delete this information from your account, open the Settings on your Apple Watch and navigate to General > Reset, then tap Erase All Content and Settings. Your content will only be erased when your Watch is paired with your iPhone.