How to restore the iPhone as a new phone with the same OS version? I have an iPhone 5s. It currently has iOS 8.4.1 on it.  I want to restore it as a new phone, but retain iOS 8.4.1.

Is this possible?  Or will iTunes force iOS 9.x onto the phone?
Settings > General > Reset > "Erase All Content and Settings"

Erase all content and settings - Apple Support