GeneXus 15 Upgrade 1 previews were part of the releasing process of Upgrade 1. They were intended to round up Upgrade 1 and facilitate adoption once released.
Upgrade 1 has been released on October 26th, 2016
https://www.genexus.com/developers/DownloadCenter?,,,4978;;
GeneXus: http://www.genexus.com/developers/RN?,0,,0;4;V15;0;1;;
GXserver: http://www.genexus.com/developers/RN?,0,,0;6;V15;0;1;;
GXflow: http://www.genexus.com/developers/RN?,0,,0;3;V15;0;1;;