Ars Technica:

Apple has begun pushing iOS 13 and watchOS 6 to supported iPhones and Apple Watches. Both updates bring substantial changes—especially iOS 13—and mark the beginning of a new annual update cycle.

While it may take some time, most users should see iOS 13 become available in the Settings apps on their iPhones or iPod touches by the end of the day today. The watchOS update will pop up in the Watch app on iPhones that are connected to an Apple Watch.

iPadOS, macOS, and tvOS releases are still on the way, though.

Take this free advice for what it’s worth: DO NOT install iOS 13 today on any devices you NEED to be stable and not buggy. Just wait until next Tuesday for iOS 13.1.