Microsoft began rolling out a third post-RTM update for.1 today, so it's now available for anyone on the Developer Preview. There are no functional changes in the update for existing handsets, but it's always a good idea to stay up-to-date.
Microsoft finalized Windows Phone 8.1 in early April and provided it to reviewers at Build and then to developers (and, obviously, enthusiasts) via the Developer Preview program. (If you have a Windows Phone 8-based handset and would like to get updated before general availability, you can check out my instructions in Get Windows Phone 8.1 as Soon as Possible.) Since then it has provided two post-RTM updates, the first in mid-May and addressing battery life issues, and the second just last week.
Windows Phone 8.1 is generally available now in the Lumia 630 and 635, neither of which is sold in the United States. And it will become available to existing Windows Phone 8 handsets through wireless carriers over the course of the summer.
As for today's update, it advances the OS to build 12397, up from 12393 in the previous update. The bad news? There's nothing new in there to really play with.
"This update enables some newly commercialized devices and tweaks a couple of new APIs, so you won't see a difference on your device," Microsoft reports. "But we encourage everyone who's on the Preview for Developers to stay current on the latest build."
If you are on the Developer Preview, navigate to Settings, Phone Update to check for updates. If you wait, you'll be prompted automatically at a later time. (My Lumia 520 actually prompted me today, which is how I found out about this in the first place.)