Open the Watch app on the iPhone, and make sure you’re in the My Watch tab (the lefthand icon from the menu along the bottom of the screen). Go to General, then scroll down and tap Reset at the bottom of the list.

Now tap ‘Erase Apple Watch Content and Settings’, the top option. You’ll be given a chance to change your mind; if you don’t want to, tap ‘Erase All Content and Settings’ to confirm.
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