How to update
You will need to set Steam to download the beta build. You can do this by locating The Flock in your library, then right-clicking on the entry, click Properties, go to the Betas tab, and select "externaltest" from the list of betas. After you close the properties box, you should see "[externaltest]" in The Flock's library entry.
Important: this update is incompatible with the current non-beta build. Make sure that everyone you're playing with are using the beta branch and are up-to-date.
What's new
There are a number of new features. Most notably, the game now has achievements and leaderboards. Currently there are 14 achievements for you to collect, and two leaderboards. Note that most of the achievements require you to be in a game of 2 or more players to unlock. In addition to that, detailed local stats are tracked, and those are synced through Steam Cloud so you'll be able to keep the same stats wherever you play. Another addition is you can now customize the game setting choices, from level to mode to various other settings.
What's fixed
Numerous issues were fixed, most notably the problem with particle effects rendering on the end of the artifact on first person view and the petrify effect sometimes spawning with the player in Angels mode.
What's broken
I'm happy to say that nothing new's broken, but the Linux version still requires a lot of work done on with regards to graphics.
More details on what changed is available on this discussion thread. Take the update for a spin, and if you have any comments, leave them below or on said thread.