How do I remove all passcodes from an iphone i must return to seller

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Restore your iPhone, iPad, or iPod to factory settings - Apple Support

It’s easy. All you need is do these steps:

1. You go to settings -> tap your name -> scroll all the way to the bottom -> tap at “sign out” button and enter your Apple ID password
2. After signing out, head to general -> reset -> reset all content and settings. Follow the instructions and you are ready to go 😉