Earlier today, Microsoft accidentally released a Windows 10 build that was meant for the Canary ring. While the build presents an all-new Start Menu, it's still something that shouldn't be on Insiders' machines. And if you're on the Slow or Release Preview rings, then you probably don't even want it.
According to a blog post from Jason Howard, there was a configuration change that caused the build to be released to multiple rings at the same time, rather than just Canary. Of course, the issue has been fixed so it won't happen again.
Jason also provided instructions for how to roll back to your previous build. After all, the Release Preview and Slow rings are meant to be on 19H1 and 19H2, respectively, so they certainly shouldn't be on a 20H1 build. The Fast ring is in better shape, since this build will just be replaced by the next one.
First, you'll want to make sure you're on build 18947. You can do this by typing the command 'winver', or going to Settings -> System -> About. If you're not on build 18947, you should go to Windows Update and make sure it's not pending.
If the update is pending, choose to pause updates for seven days. After that, it will cancel the update, so you can reboot and unpause updates.
If you are on build 18947, you can roll back to the build you should be on. You can do this by going to Settings -> Update & security -> Recovery -> Go back to the previous version of Windows 10. Follow the onscreen instructions to roll back.
Note that the only way to get updates moving forward from this build is to be on the Fast ring. So if you're on any other ring, you'll certainly want to roll back.