How to downgrade your Nokia Lumia from Windows Phone 8.1 Preview (and get the official update)
The two prerequisites are: you need a PC running Windows 7 or higher, and (of course) a Nokia Lumia running the Windows Phone 8.1 Preview. To start, always be sure to make a backup of your data by going to Settings ==> Backup ==> App + settings ==> Back up now. Once that bit of housekeeping is done, you'll need to download the appropriate Nokia Recovery Software Tool (links below). From there, it is pretty easy. Just, install the program, open it up, connect your phone to your PC via USB, do another backup (just in case), select the Windows Phone 8.0 software, and follow the instructions. The program will download the software and flash it to your phone (erasing everything in the process, so be sure to make those backups).
Once your phone is back on Windows Phone 8.0, it will just be a matter of checking for the official Cyan update as it becomes available for your Nokia. Seeing as how the Lumia 1520 update just started rolling out, it may not be available right away. Lastly, Microsoft could reverse its stance on providing updates directly from the preview software, so you can wait and see how it goes. But, there's no guarantee if or when that might happen.