Windows Admin Center version 1809.5 is a cumulative update to our 1809 GA release in September which includes various quality and functional improvements and bug fixes throughout the product. This month, Windows Insiders get early access to our 1809.5 release before it is made generally available.
See the official blog post for details on changes and how to download the new release: https://blogs.windows.com/windowsexperience/2018/12/11/announcing-windows-admin-center-1809-5-insider-preview