To get the beta you'll need to join Apple's Beta Software Program. You can do that here. The second public beta corresponds to the third developer beta which was seeded to developers on Monday.
The new beta brings a few changes including new OS X Server and Dropbox locations in the Files app, additional TV providers, swipe to close apps in the iPad app switcher, an indicator on how many messages are being uploaded to iMessages in the Cloud, and more.
To learn more about the new features in iOS 11, click here.