Apple released, and subsequently pulled iOS 12 beta 7 after developers began to report extremely poor performance once the update was installed. Today, Apple seeded the eight beta for iOS 12 to developers.
You can download it using 3uTools, users who restored to beta 7 are recommended to upgrade to this beta version.
As always, please back up your device before any change. If you meet any flashing error, you could view tutorial Roundup: Error Codes in iTunes/3uTools for reference. In exceptional cases, you could contact the 3u Team for further support.
Step 1 Connect your device to PC with a USB cable and launch 3uTools.
Step 2 Head to Flash & JB → Easy Flash, select the iOS 12.0beta8 firmware. If you want to save data, please click the checkbox “Retain User’s Data While Flashing”. Click “Flash” to start downloading iOS 12 beta 8 firmware.
Step 3 Please do not disconnect your iDevice and wait patiently until the process is finished, it would probably take 5 to 10 minutes. And after that, your device will be restored to iOS 12 beta 8.