This one is actually the fifth update that we've done since beta started three days ago. However, this is the first one that is getting its own post; I was just in too much of a hurry with everything going on to post about the other releases, despite the fact that they had significant improvements and polish.
This new version includes a lot of top-requested features, such as: not losing your inventory when you die on the overworld; some better handling of some specific annoying cases with the follower player-2-standing; some improvements to the visual clarity of the bonus objectives completion status; and having the player characters slide around object corners rather than getting stuck on them.
There are also a bunch of new music tracks in this one, and a bunch of levels that had only had ambient sound effects before now have their music tracks properly set along with that. There are also two new weapons that are largely overworld-specific.
This is a standard update that you can download through the in-game updater itself, if you already have any version of the game. When you launch the game, hit the big "Play" button and then you'll see the notice of the update having been found if you're connected to the Internet at the time.