Apple has made available the fifth beta of Mac OS X El Capitan to developers for testing purposes. It’s available through the Software Update section of the Mac App Store and through the Apple Developer Center (https://developer.apple.com).
The new beta focuses on bug fixes, not new features. OS X 10.11 El Capitan is currently available to both registered developers and public beta testers. Apple plans to release El Capitan to the public in the fall.