iOS 11 is available on Tuesday September 19th, and if your device is compatible, you can go ahead and update, by just tapping the button in Settings>General>Software Update. If all goes well (and it should), then you will wait for a while as the update downloads and installs, then your iPhone or iPad will restart into the new version of iOS, with all the cool goodies it brings.
But things sometimes can go wrong, so it pays to take a few precautions. You might also like to take the opportunity to clean up your device a little. Here’s how to prepare your iDevice for iOS 11.