This software is still in development, so we don’t recommend you run it on a production site. Consider setting up a test site just to play with the new version. To test WordPress 4.9, try the WordPress Beta Tester plugin (you’ll want “bleeding edge nightlies”). Or you can download the beta here (zip).
For more information on what’s new in 4.9, check out the Beta 1 blog post. Since then, we’ve made 70 changes in Beta 2.
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If you think you’ve found a bug, you can post to the Alpha/Beta area in the support forums. We’d love to hear from you! If you’re comfortable writing a reproducible bug report, file one on WordPress Trac, where you can also find a list of known bugs.