How to Erase an 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, you can go to the Apple website from another computer (https://support.apple.com/apple-id) and try to recover your Apple ID.

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.