The second beta version of macOS Sierra is now available for developers. Those developers interested in testing the update can either grab it from Apple's Developer Download site, or through the Mac App Store's Updates section on a machine running the previous beta.
macOS Sierra will bring new features and improvements to the Mac user experience when it launches this fall. In addition to the notable name change, Sierra will see the debut of Siri on the Mac, along with Continuity improvements like Universal Clipboard, and Apple Pay on the web.
Apple occasionally offers updates to iOS 10, watchOS 3, tvOS 10, and macOS Sierra as closed developer previews or public betas for iPhone, iPad, and Mac. (Sadly, no public betas for the Apple Watch or tvOS.) While the betas contain new features, they also contain pre-release bugs that can prevent the normal use of your iPhone, iPad, Apple Watch, Apple TV, or Mac, and are not intended for everyday use on a primary device. That's why we strongly recommend staying away from developer previews unless you need them for software development, and using the public betas with caution. If you depend on your devices, wait for the final release.