Erase all content and settings from iPhone
When you delete data, it’s no longer accessible through the iPhone interface, but it isn’t erased from iPhone storage. To remove all of your content and settings from storage, erase iPhone. For example, erase iPhone before you sell it or give it away. If you want to save your content and settings, back up using iCloud or iTunes before erasing iPhone.
Go to Settings
> General > Reset.
If you’re asked to enter your passcode and you’ve forgotten it, see Reset the passcode. If you’re asked to enter your Apple ID password and you’ve forgotten it, see the Recover your Apple ID website.
Tap Erase All Content and Settings.
When iPhone restarts with all content and settings erased, the setup assistant gives you the option to either set up iPhone as new or restore it from an iCloud or iTunes backup. (See Turn on and set up iPhone.)