To try out the public betas, you need to sign up for the Apple Beta Software Program.
The Apple Beta Software Program lets users try out pre-release software. The feedback you provide on quality and usability helps us identify issues, fix them, and make Apple software even better. Please note that since the public beta software has not yet been commercially released by Apple, it may contain errors or inaccuracies and may not function as well as commercially released software. Be sure to back up your iOS device with iTunes and your Mac using Time Machine before installing beta software. Since Apple TV purchases and data are stored in the cloud, there’s no need to back up your Apple TV. Install the beta software only on non-production devices that are not business critical. We strongly recommend installing on a secondary system or device, or on a secondary partition on your Mac.
Click here to learn more about what's new in iOS 11. You can sign up for the beta program at the link below...