In the past, only developers can participate in testing Apple's beta software. But that has already changed allowing public users as well to take part in testing Apple's operating system. If you haven't signed up to the program, its as simple as going to Apple's Beta Software Program site and enrolling your Mac or iOS device. You need to be extra cautious though since the software is not yet final and there are definitely a lot of bugs. For the Mac, you might want to create a partition on your disk to separate the macOS beta with your working software. For your iOS device, you might want to consider using another device, for example an iPad, instead of your working iPhone. In my case, I might install the macOS Sierra beta on an external hard drive and the iOS 10 beta on my iPhone 6. I have enough experience already handling these kinds of software release so I'm comfortable with the risks involved. An important tip that I always share is to ensure that you have a clean backup of your devices prior to installing beta software. Another thing that I learned before for iOS is to encrypt the backup to ensure that Health data can be retrieved when you decide to restore stable software. My one year's worth of Health data was wiped out before since I don't encrypt my backup. You won't miss this encryption as its available as a checkbox in the Summary pane of iTunes. The public beta is expected to be released this month so if you wan't to get a hands on of macOS Sierra and iOS 10 before its final release, now is your chance so start signing up!
You can signup here --> Apple Beta Software Program