How to Force an iPhone to Restore
The iPhone's reset menu can be used if you are unable to access a computer to restore the device. It is irreversible once the process begins. After it's restored, it can be sold or set up as a new device in iTunes, as all of your previous information will no longer be available.
Tap the "Home" button on the iPhone and unlock the screen.
Click "Settings" and "General."
Scroll down to the last tab of the menu and click "Reset."
Click "Erase All Content and Settings." Click "Erase iPhone."