http://www.virtualradarserver.co.uk/Preview.aspx.
The page also gives details about the changes implemented in the preview version since the last proper release.
Preview installers overwrite your current installation of Virtual Radar Server. If you want to get back to a stable release then you should just be able to run the release version's installer, you shouldn't need to uninstall first. However there is a possibility that a preview installer could corrupt the configuration files and make it impossible to roll back to a release build, so if you are installing a preview version over the top of a release version then you might want to make a backup copy of your configuration before you run the installer
You can find your configuration files by going to Help | About and clicking the link for the configuration folder on that screen. The important files to make a copy of are Configuration.xml and Users.*