This one is mostly a bunch of bugfixes to deal with things that made it into the 6.0 official. None of them really critical, though the scapegoat bug (i.e. the AI finds a way to counter the Martyr) was pretty hilarious, and these are good to get fixed in any event.
This is a standard update that you can download through the in-game updater itself, if you already have 4.000 or later. When you launch the game, you'll see the notice of the update having been found if you're connected to the Internet at the time. If you don't have 4.000 or later, you can download that here.