We’ve previously reported here how those who had opted to install the Developer’s Preview of Windows Phone 8.1 had been unable to install the Lumia Cyan firmware. True to its word, Microsoft has (finally!) resolved whatever the glitch was and Lumia Cyan can now be installed. The usual process should show an update ready to install: settings > phone update
As usual, it is recommended that you backup first. I’ve got it working on my Lumia 1520, but rather annoyingly the snazzy new ringtones that can be found preinstalled on a Lumia 930 are not there.