Developer Beta 2 is out for iOS 10, macOS Sierra, watchOS 3, and tvOS 10
Anyone registered in Apple’s $99/year Developer Program can get access to the new betas from the Downloads section of the developer site, or by upgrading devices directly. iOS 10, for example, is available as a configuration profile, and once you install that on a registered iPhone or iPad, updates come in over-the-air in Settings > General > Software Update.
watchOS 3 is updated via the Watch app on a paired iPhone running iOS 10. tvOS 10 is installed by connecting a fourth-gen Apple TV to a Mac with a USB cable and updating via iTunes, or you could try a workaround using Dropbox. To get macOS Sierra, developers simply enter a personalized redemption code in the Mac App Store, and then further updates come that way. Of course, you should always back up before upgrading.
Why this matters: Public betas of macOS Sierra and iOS 10 are expected sometime in July. Last year the first public beta corresponded to the third developer beta, so we might still be a little ways out. Or Apple could decide that Developer Beta 2 is stable enough to unleash to beta testers, and we could see it fairly soon. If you’re interested in helping Apple kick the tires of iOS 10 and macOS Sierra, you can sign up for the public beta and be notified as soon as it’s ready.