To get the beta you'll need to join Apple's Beta Software Program. You can do that here. The second public beta corresponds to the third developer beta which was seeded to developers on Monday.
You can learn about what's new in macOS High Sierra by clicking here.
The macOS 10.13 Preview supports
• MacBook or iMac: Late 2009 or newer
• MacBook Air, MacBook Pro, Mac mini, or Mac Pro: 2010 or newer
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