At the moment only developers have access to the beta software of Apple’s products (iOS 10, macOS Sierra, and watchOS 3). But if you want to get your hands on the beta build and familiarize yourself with the new features, next month Apple will be opening iOS 10 and macOS Sierra to public testers. To be one of these testers, all you have to do is sign up on Apple’s dedicated page for the Apple Beta Software Program using your Apple ID. If you don’t have one of those, you can create one for free. After you agree with Apple’s terms and conditions, you can then enroll your device to receive notifications of new updates and releases for download.
Once you’re a member, you just have to make sure you have the latest version of iTunes on your PC and when the beta is released in July, just connect your enrolled iOS device to the computer and open iTunes or run Software Update. For macOS Sierra testers, you can get the beta software from the Mac App Store. A quick reminder, though, and Apple reminds you of this is that you shouldn’t install beta builds in your daily drivers or work devices as these builds come with risk. Beta tests are meant to work out the issues these OSes have so putting it in something you need daily could pose a problem.
Source: Apple Insider