The second phase of the Beta starts tomorrow, August 16th. All fans who have preordered the game via a regular retailer or won a key in a community contest will be able to play. If you played the first phase of the Beta, you will only need to launch the Beta when it reopens and than the content will be updated automatically.

The changes made after the last Beta round can be found on this page and I gather that the official Beta page will be updated with even more relevant info as well.