Here's one way to reset your iPhone, simply hold down the sleep/wake button. You may find this at the top right of the iPhone and the home button, which can be found in the bottom centre of the face; at the same time for anywhere between 10-15 seconds, the screen will then turn black and the Apple logo will reappear, that means the iPhone is rebooting. Just a heads up, resetting the device will not delete any data or default any of the settings. So your iPhone is safe to perform this trick.