https://arstechnica.com/gadgets/2017/09/ios-11-is-now-available-for-download-on-supported-devices/

As expected, Apple has begun rolling iOS 11 out to iPhones and iPads today in most regions. You can probably download it right now, and, if not, you’ll be able to fetch it very shortly.

Devices as far back as the iPhone 5S, the iPad Air, and the iPad mini 2 can update to iOS 11. But the iPhone 5 and 5C, as well as the fourth generation iPad and the very first iPad mini, are not supported by iOS 11.