This one is the first of two releases for today. It includes one critical bugfix where you couldn't always pick up items correctly in the last version, which is the main reason this was pushed out. But it also has other bugfixes and tweaks, a new cutscene, and four new music tracks. Also improvements to how well the grays are stunned by being shot with small caliber bullets.
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.