If you own a Windows Phone 8.1 device and are waiting for the official over-the-air update to Windows 10 Mobile, you may have to download a separate firmware update for that phone beforehand.
According to what a Microsoft spokesperson told Neowin:
As we have previously reported, carrier approval will be needed before older Windows Phone 8.1 products can receive an over-the-air Windows 10 Mobile software update. However, once that is completed, Microsoft looks like it can then push out software updates to the OS without carrier approval.
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