AT&T announced that their Lumia 1520 is now receiving the Lumia Cyan and Windows Phone 8.1 updates. However, it quickly became evident that those with the Preview for Developers installed on their Lumia 1520s did not have an update, but those with Windows Phone 8.0 did



From WPCentral:

As such, those who are on the Preview and who still want the Lumia Cyan update need to downgrade their OS first. This is still Microsoft's position on the situation.

The original issue stems from those who were using the Preview for Developers version of Windows Phone 8.1 and who have BitLocker encryption enabled (through their employer). Although those affected are a small minority of users, when they installed the official 8.1 update, it caused the phone to lockup. As a precaution, Microsoft halted the Preview program for everyone.

  Windows Phone 8.1 Preview users still need to downgrade their OS in order to get Lumia Cyan