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.
macOS High Sierra 10.13.5 once again introduces support for Messages on iCloud, a feature that was present in macOS High Sierra 10.13.4 betas but pulled ahead of the release of the updated software.
Messages on iCloud is designed to let you store iMessages in iCloud, keeping your incoming messages synced across devices and allowing you to save valuable storage space on your devices.
No other major outward-facing changes were discovered in the first developer beta, but the update likely includes bug fixes and improvements to address issues discovered in macOS High Sierra 10.13.4.