Steam will usually automatically put workshop updates into your scheduled downloads list, which it puts on pause if you are playing any games through Steam. But sometimes Steam is stupid and just won't do anything. In that case I believe unsubscribing from Buak, waiting a few minutes for Steam to uninstall Buak (which I don't think it tells you that it is doing or when it is done), then resubscribing to Buak and should make Steam wake up and download the newest version.

To the best of my knowledge, unsubscribing from Buak should NOT delete your single player save file for Buak if you happen to be playing single player.