The update of macOS Mojave can be downloaded using the software update mechanism in the Mac App Store after downloading the appropriate profile from Apple's beta test site. Apple's beta test site gives users access to iOS, macOS and tvOS beta.
macOS Mojave 1
The update also includes support for Touch ID enabled Safari AutoFill and automatic Dark Mode themes in Safari. That means if you have Dark Mode enabled on macOS Mojave when you visit a site with a dark theme, it will be activated automatically. You can see a demonstration of the feature here.
macOS Mojave 10.14.4 will probably be in beta testing in the next few weeks, when Apple prepares features and prepares errors. After that, it will see a release along with iOS 12.2, watchOS 5.2 and tvOS 12.2.