In order to be able to install latest iOS public beta, you need to be a part of the “Apple Beta Software Program.”
How to Enroll Apple Beta Software Program
Here are the steps to enroll yourself in the program and install iOS 12:
Step 1: Open Safari on your iOS Device and visit: https://beta.apple.com/sp/betaprogram/
Step 2: Click on ‘Sign up’.
Step 3: Sign in using your Apple ID
Step 4: Select ‘Accept’.
Step 5: Select the tab iOS.
Step 6: Select Enroll your Device link.
Step 7: Scroll down and click on Download profile.
Step 8: Tap on ‘Allow’.
Step 9: Select ‘Install’ from top right corner. To confirm, select ‘Install’ again on the pop-up box for confirmation.
Step 10: Restart the device when prompted after this screen.
How to Install iOS 12 public beta
Step 11: Go to Software Update and download iOS 12 Beta
Step 12: Tap ‘Install Now’.
Congratulations! When you see this screen, your device is on iOS 12.