How to Remove Information on an iPhone
If you are selling your old iPhone, you should clear all of your information before handing it over to a new owner. Although you may not see any information on your phone after deleting your email accounts, contacts and applications, your iPhone stores private data that can be accessed with a little investigating. A full erase will remove your information and settings permanently, so do not do it unless you are fully restoring your phone to its original settings. It only takes a few minutes and will prevent your information from being accessed by others.
Tap the "Settings" icon on your iPhone.
Tap on "General" to open the phones general information section.
Scroll to the bottom of the screen until you see "Reset."
Tap on "Reset" to view the "Reset" options. This will not reset your phone. It will display the different options you may choose from.
Tap on "Erase All Content and Settings" one time to view the next screen.
Tap on "Erase iPhone." The iPhone will request that you tap "Erase iPhone" a second time to confirm you would like to erase all information from your iPhone.