1. Connect iPhone to your computer
2. In iTunes, select iPhone in the Devices list, then click Summary at the top of the screen.
3. Click Check for Update iTunes tells you if there’s a newer version of the iPhone software available.
4. Click Restore. Follow the onscreen instructions to complete the restore process. When restoring, it is recom­mended that you back up your iPhone when prompted.

When the iPhone software has been restored, you can either set it up as a new iPhone, or restore your music, videos, app data, and other content from a backup.

After you restore from a backup, previous data is no longer accessible through the iPhone user interface, but it isn’t erased from iPhone.