If you're new to the macOS High Sierra Beta or Public Beta testing, you'll have to run the macOS Developer Beta Access Utility to download the latest macOS High Sierra 10.13 beta.
Registered developers running a previous beta can install it right now from the Updates pane in the Mac App Store application. Tagged as build 17E190a, the macOS High Sierra 10.13.4 beta 6 is out and brings more stability, security, and compatibility improvements for your Mac. Following last week's sixth iOS 11.3 and watchOS 4.3 beta updates, Apple released on Monday the macOS High Sierra 10.13.4 beta 6 development milestone to registered developers, along with other components.