The new beta is available through the Software Update mechanism in the Mac App Store for those who are enrolled in Apple’s beta testing program. Those wishing to join the program can sign up on Apple’s beta testing website.
Like many of the previous updates to the OS X 10.11 El Capitan operating system, OS X 10.11.6 appears to be minor in scale, focusing on under-the-hood bug fixes, security enhancements, and performance improvements. No obvious outward-facing changes or bug fixes were discovered in the first developer beta.
Source: Mac Rumors