Beta testers: to the App Store! Apple released an update to the El Capitan public beta today, the third version. If three is your lucky number, we've got instructions on how to sign up for the beta program, and a how-to on filing feedback when you encounter bugs. Once you're enrolled, the new beta version is available in the Updates tab of the Mac App Store.
iOS 9 is also in public beta, but wasn't updated today, as of this writing. Both iOS 9 and OS X 10.11 El Capitan are scheduled to launch in the fall, as free updates.
Apple recommends you install the public betas of its operating systems on a secondary Mac, or at least a separate partition. If something goes wrong, it's not that hard to revert to Yosemite, but it should go without saying that you should always back up your data just in case.
Not interested? You can still get a sneak peek at some of El Capitan's new features by installing similar third-party apps in Yosemite.