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I have enabled Ribbon UI and it looks like ribbon ui build in nineties. There is no obvious way to revert the change.

I think you can restore the default from the main menu?

View -> Toolbar Layout -> Default

It was not obvious as the main menu was hidden. There is a button (top-right corner) to show main menu.

This is why it's still an exerimental feature, and the more testing and feedback the design community gets, the better!