Beta testers who have signed up for Apple's beta testing program will receive the iOS 11 beta update over-the-air after installing the proper certificate on an iOS device.
https://www.imore.com/how-download-ios-11-public-beta-1-your-iphone-or-ipad
https://www.imore.com/how-download-ios-11-public-beta-1-your-iphone-or-ipad