This guide will show you how to install the iOS 11 Beta Profile on your device.
iOS Beta Profile: Download
watchOS Beta Profile: Download
tvOS Beta Profile: Download
macOS Beta Profile: Download
Source: Apple Betas Download Center
Scroll until you find the iOS Beta Profile and click the blue Download button.
Now a prompt will appear, click iPhone if the iOS Beta Software Profile is selected. If you want the watchOS Beta Software, download that instead.
Tap Install in the Settings app, then enter your passcode to continue.
Agree to the iOS Beta message from Apple so the process can be completed.
Now restart your device and check for updates! You should now see the iOS 11 beta software available for download.