Apple has released a golden master (GM) candidate of macOS High Sierra to developers and public beta testers A golden master is a final version of software ready for release to manufacturing.
The macOS High Sierra GM can be downloaded from the Apple Developer Center or over-the-air using the Software Update mechanism in the Mac App Store for those in Apple’s Public Beta Testing Program. The finished version of the operating system update is due Tuesday, September 20. macOS High Sierra will run on all machines that are compatible with macOS Sierra.