The Safari Technology Preview update is available through the Software Update mechanism in the Mac App Store to anyone who has downloaded the browser. Versions are available for developers running both macOS Sierra and OS X El Capitan. Full release notes for the update are available on the Safari Technology Preview website.
Apple's goal with Safari Technology Preview is to gather feedback from developers and users on its browser development process. Safari Technology Preview can be run side-by-side with the existing Safari browser and while designed for developers, it does not require a developer account to download.