Understand the macOS Beta Installation Process
If you are a developer, you can download a small file from the Developer website that will allow you to see the macOS High Sierra beta in the Mac App Store. Public beta users will get a similar file when it is available. If you can't wait, you can download the file from the beta.applebetas website.
Once you have the file installed on your Mac, you will see macOS High Sierra in the Mac App Store available to download and install like a normal macOS update.
If you don't see the macOS High Sierra beta download option in the App Store, restart your Mac and check again. This may take a very long time to download, so you should set aside a good deal of time and do this at home when you don't need to go anywhere soon.
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