You need to reset your iPhone to factory settings, if you want it to appear as fresh and brand new. You should reset iPhone to factory settings, if you are planning to sell or transfer ownership of your iPhone. It is also an ideal process for troubleshooting technique. You should backup the iPhone manually or to iCloud before you reset iPhone to factory settings.
How to Reset iPhone to Factory Settings:
First of all tap on “Settings” and then tap on “General”
Scroll down to the bottom of General and tap on “Reset”
Tap on “Erase All Content and Settings”
Enter the iPhone passcode, and confirm the reset by tapping on “Erase iPhone”
Depending on the model, resting iPhone to factory settings can take a few minutes to more than an hour to restore the device completely.