Beta testers who have signed up for Apple's beta testing program will be able to download the new macOS High Sierra beta through the Software Update mechanism in the Mac App Store.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to iOS, macOS, and tvOS betas.
No notable features or changes were discovered in the first two betas of macOS 10.13.6 provided to developers, suggesting the new software focuses on bug fixes and other under-the-hood updates.
macOS 10.13.6 is likely to be one of the last updates to macOS High Sierra, as Apple is transitioning to macOS Mojave. macOS Mojave was unveiled last week at the Worldwide Developers Conference.