Q: New iPhone has iOS 8.2, but my backup is 8.4. How to I update the phone as it is in setup mode

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Follow these steps to update your iPhone to iOS 8.4 and restore that backup:

1. Set up your new iPhone skipping the backup restore step. You have to set it up as a new device.
2. Go to Settings -> General -> Software Update, and update to iOS 8.4.
3. After updating, go to Settings -> General -> Reset -> Erase All Content and Settings, and follow the steps to restore your backup.

If restoring from an iTunes or iCloud backup requires a newer version of iOS - Apple Support