GeneXus 15 Upgrade 2 previews were part of the releasing process of Upgrade 2. They were intended to round up Upgrade 2 and facilitate adoption once released.
Upgrade 2 has been released on 22nd December, 2016
https://www.genexus.com/developers/DownloadCenter?,,,4978;;
GeneXus: http://www.genexus.com/developers/RN?,0,,0;4;V15;1;2;;
GXserver: http://www.genexus.com/developers/RN?,0,,0;6;V15;1;2;;
GXflow: http://www.genexus.com/developers/RN?,0,,0;3;V15;1;2;;