Unlike apps that you download from the App Store, you can't simply delete and reinstall an iPhone's native apps if they begin causing you problems. If you need to restore any of these apps, like Notes or Contacts, you have to completely restore the iPhone to its factory default settings. Unfortunately, the process will delete all of your data and settings. You should back up the iPhone before restoring it, but keep in mind that reinstalling the backup may just recreate whatever problem you were having that made you need to restore the iPhone in the first place.
Restoring Without iTunes
Connect the iPhone to a power outlet using the AC adapter it came with. Ensure it remains connected throughout the process; if the phone runs out of power before it finishes restoring the operating system, you may be unable to boot it up again.
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Back up your iPhone's data to iCloud if you want to restore your data and settings after restoring the iPhone. To do this, launch "Settings" from the home screen and tap "iCloud." Set up your iCloud account using the onscreen instructions if you haven't done so already, then tap the "Storage & Backup" option. Tap the "iCloud Backup" button to turn it on, then scroll down and tap "Back Up Now." This may take over an hour if you have a lot of data to back up.
Tap the "Settings" icon on the iPhone home screen. Select "General" and tap "Reset." Select the "Erase All Content and Settings" option. This process should only take a few minutes if you have an iPhone 3GS or later. The iPhone deletes the encryption key protecting its data. Older models may take up to a few hours since the iPhone overwrites all of its data.
Unlock the iPhone after it restarts. Follow the onscreen instructions to set up the iPhone as a new device. Only the factory default apps are installed. Alternatively, you can restore from the iCloud backup to restore your purchased apps and personal data as well.
Restoring With iTunes
Download and install iTunes if you haven't done so already. Launch iTunes and connect the iPhone to a USB port on the computer using its adapter cable.
Back up the iPhone, if desired, by clicking the "File" menu, selecting "Devices" and clicking "Backup."
Click the "iPhone" button in the upper right corner of the iTunes window. Click the "Summary" tab and then click the "Restore" button. A dialog window opens asking you to confirm your decision. Click "Restore."
Unlock the iPhone after it restarts. Follow the onscreen instructions to set up the iPhone as a new device or use the iTunes backup to retrieve your data and settings.