There is an option in the setup window.
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As far as I know, there were some problems on (dev) beta 2, related to the new file system, so people had troubles downgrading to 10.12. The first public beta is actually a new build which was released earlier today, so probably this bug is fixed. I don't think Apple will release a public beta if it cannot be downgraded. There is a manual how to downgrade on Apple's website (
https://beta.apple.com/sp/betaprogram/restore).
Although, if you really rely on the machine you want to install High Sierra Public Beta on, I would suggest you to wait 'till later betas or 'till the official release. The current beta is still a little bit sluggish. What's more, as it's not yet optimised for a public release, it uses significant energy, which leads to shorter battery life and overheating (at least on MacBooks).
And with macOS 10.12.6 coming in the following week or two, there'll be something new to play with, even if it's not as exciting as 10.13.