The first update to Windows Phone 8.1 has rolled out today and is the first since the developer preview was released into the wild on April 14th.
You need to make sure that you are still signed up to the Preview for Developers program, have the app installed and are logged in via your Microsoft account to receive the update.
Joe Belfiore has said that the update will improve the battery life (which a lot of users have been experiencing issues with, myself included) and bug fixes.
You can get the update the normal way from settings -> phone update -> check for update and it will update you to OS version 8.10.12382.878.
Thanks, Windows Phone Central