To let WSUS to upgrade (and not update) the Windows 10 clients you need to:
1- Install this update if not already installed : KB 3095113 (http://blogs.technet.com/b/wsus/archive/2015/12/04/important-update-for-wsus-4-0-kb-3095113.aspx)
2- Add the .esd – application/octet-stream mime type on the iis server of the wsus server :
