This is the best way to get the iOS 12 Beta On a compatible phone before everyone..
*Please Read The Highlighted Text Before Starting For The Update.
Create A Backup:
- Plug in a compatible iPhone or iPad into your Mac or Windows PC.
- Launch iTunes.
- Click on the iPhone or iPad icon in the menu bar when it appears.
- Make sure backup is set to This Computer. (And enable Encrypted backup — it’s worth it!)
- Click on Back Up Now.
- Back up Apps, if asked
- Press Command, to open Preferences.
- Click on the Devices tab.
- Control-click on your Backup and choose Archive.
Registering For The Beta:
- Go to developer.apple.com on your iPhone or iPad.
- Click on the “Download New Betas” tab.
- Log in with your developer account’s username and password.
- Go to the iOS 12 beta section and tap the download. This will start the process of downloading the iOS beta software profile on your iPhone.
- The Beta software profile will appear on your phone. Tap Install. You may need to enter your Passcode.
- Agree to the Consent form and press Install once in the upper right corner, and once at the bottom of the screen.
- Press Restart to apply the changes to your iPhone.
Installing The Beta:
- Launch Settings from your Home screen, tap on General, then tap on Software Update.
- Once the update appears, tap on Download and Install.
- Enter your Passcode.
- Tap Agree to the Terms and Conditions.
- Tap Agree again to confirm.
And After The Restart You Are All Set To Use The iOS 12 Beta..
*Installing Developer Beta May Cause Accidental Shutdown Or May Result In Damaged Battery Life Please Make Sure You Do Not Install It On A Primary Device. **No Members Of AppleSummit Team Are Responsible For Loss Of Data, Bugs Or Lag Due To The Beta.
Source: Edited iOS 11 Beta From Apple.Com And Updated The Links For iOS 12