To get Unreal Engine 4.19 Preview 1, head to the Library section on the launcher, select “Add Versions” and choose 4.19 Preview 1.
A more inclusive list of upcoming changes in the build has been posted on our forums. Please keep in mind that Preview releases are intended only to provide a sample of what is going to be released in the update and are not production-ready. We recommend waiting for the full release of 4.19 before updating your projects to the new engine version. For now, please use copies of your projects to test new features.