First navigate to Settings > General > Software Update to check if you have the iOS 11 public beta. If you see a message that reads "Your software is up to date" then you need to re-enroll in the Beta Program. Make sure you have upgraded to iOS 10.3.3 then follow these steps:
- Navigate to beta.apple.com/profile on the device you want to update.
- Sign in using your Apple ID.
- Download the configuration profile to your device.
- Restart your device.
- Navigate to Settings > General > Software Update and install iOS 11.