This one addresses two minor issues, and a third issue that could make the end of the game excessively hard to complete under certain circumstances (on higher difficulties in particular).
This is a standard update that you can download through the in-game updater itself, if you already have any version of the game. If you have the beta on Steam, it will automatically update for you. 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 the standalone game, you can download that here. If you already own the first game, just use your existing license key to unlock the sequel for free!