Apple unveiled the new iOS 10 and macOS Sierra at the annual WWDC in June. Registered developers have been able to test the developers' preview builds for a month, but now Apple invites the public to join them.
As of today Apple is opening both operating systems to the public for testing. The iOS 10 Public Beta is now available for download on compatible iPhones, iPads and iPods. But first you need to enroll your device into the Beta program from here.
iOS 10 promises lots of new features and may turn out to be one of the biggest iOS updates to date.
macOS Sierra is also available for the public to test. Just as with iOS, you need to enroll your Mac into the Beta program and then install Sierra.
The first public beta builds should be more stable than the developer ones, but this doesn't mean you should install them on your primary devices. It's risky, and eventually, you will have to restore or re-install your OS with the retail build in September.