AT&T rolls out Lumia Cyan and Windows Phone 8.1 for the Nokia Lumia 925 and Lumia 520
If you happen to be rocking the developer preview of Windows Phone 8.1, do know that the phone will not auto-update to the regular version. You need to roll-back to 8.0 and start the Lumia Cyan update from there. Check this article for info on how it's done.
If you don't own one of the newer Microsoft / Nokia devices, don't fret. It's believed that the update will eventually be rolled out to older devices, such as Lumia 820 and HTC 8X, as well. Alternatively, if you are in the market for a handset preloaded with WP 8.1 – you can check out the new Lumia 635. Or, if you are feeling lucky, you can try and enter Microsoft's giveaway for the aforementioned phone.
source: Windows Phone Daily