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How to go back to a previous version of iOS on an iPhone?
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Go Back to a Previous Version of iOS on an iPhone
1. Download the signed IPSW file for the version you want to install.
2. Back up your iPhone.
3. Disable Find My iPhone.
4. Connect your iPhone to your computer.
5. Turn off your iPhone.
6. Boot your iPhone into recovery mode.
7. Select your iPhone in iTunes or Finder.
8. Hold Alt (PC) or Option (Mac) as you click Restore.
9. Select the IPSW file and click Open.
10. Click Restore and follow the on-screen instructions.