The preview build is available to download from the Epic Games launcher and GitHub. You may have to update or restart the launcher for the new build to appear.
Please note that the recently announced addition of Epic Developer Assistant is not included in the preview. It will be ship with the full release of version 5.7.0.
⚠️ Preview builds are still under heavy and active development, and are therefore considered unstable. If you want to test an existing project, I highly recommend making a copy first.