It's only been four days since Apple seeded iOS 10 Developer Beta 6 and Public Beta 5, but the company today seeded the next iteration of its future mobile OS. While it's not uncommon for beta builds to be more frequent as we draw closer to its release, normally Apple also offers builds of macOS, tvOS, and watchOS as well, which it did not do today.
As you might guess, the update is fairly small (assuming that you're already running the previous beta), coming in at about 60.5MB. The release notes aren't specific about what's in this build or why it was released when and how it was, but it's likely that Apple found a bug that it needed to squash.
Launching in September alongside the next iPhone, iOS 10 will contain a number of new features and a refreshed experience with the OS. The lock screen experience is completely revamped, and you'll find that many stock apps have been overhauled as well, such as Music, News, and Maps. Some are now allowing third party extensions, such as Messages.
If you want to sign up for Apple's Public Beta program, you can do it here. Once you do, you'll be able to grab the latest builds of iOS 10 and/or macOS 10.12 Sierra. Sadly, there are no public betas of watchOS 3 or tvOS 10.