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 iPhone before erasing it.
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, you have the option to set up iPhone as new or restore it from a backup. (See Turn on and set up iPhone.)