The OS X 10.11.6 update can be downloaded through the Apple Developer Center or the Software Update mechanism in the Mac App Store.
OS X 10.11.6, like OS X 10.11.5 and the El Capitan updates that came before it, focuses mainly on performance improvements and bug fixes rather than outward-facing changes. According to Apple's release notes, the update improves the stability, compatibility, and security of Macs.
No obvious changes or bug fixes were found in the first two betas, but we'll update this post should anything new be discovered in the third beta. Apple has also started testing macOS Sierra, the next-generation Mac operating system set to be released this fall.