iOS 12 is beta version,so it have some bugs.If you want to backup you device,you can use iTunes to backup.
you can use your mobile device directly to update,below is step :
1.Download iOS 12 beta Software Profile.
you can use Safari to download profile
2.Open website and click allow to navigate to Settings App, and install software profile.
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3.After install profile success,restart your device.
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4.In Settings-General-Software Update,you can access iOS 12 beta OTA update package.
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5.After download system package success,click install directly and just wait some time to update your system.
