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- Assuming you've already backed up your iPhone as mentioned above, you've downloaded the appropriate version of iOS 8 for your iPhone, I've told you your device is registered, proceed to unpacking/mounting the beta image by double-clicking on the .dmg file.
- Plug your iPhone into your computer and open iTunes.
- Open the iPhone page in iTunes. Hold down Alt/Option (on your Mac keyboard; hold down Shift if on Windows) and click on Restore iPhone.... Now select the .ipsw beta file within the mounted dmg and select Open. Allow the restore to complete.
- After the restore has completed and the iPhone has reconnected to iTunes, restore from a backup, if you desire.
If you're using iTunes for backup, choose the Restore Backup... option in iTunes. Select the backup you created prior to installing the iOS 8 beta. This will restore your iPhone information. If you're using iCloud for backup, use the Restore from iCloud Backup option after iOS 8 installation is complete and the device reboots into iOS 8.
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