In order for Apple devices to be sent to Apple for Repair / Replacement, they must first be disconnected from the iCloud service. To do so:
- First make sure your device has been shut down.
- Login with your Apple ID at http://iCloud.com
- Choose "Find iPhone"
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From the dropdown called "All Devices" on the center of the screen, click on the device you have powered down:
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Select "Remove from Account"
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Confirm the removal of the device.