While we make every effort to support older phones and older versions of iOS, some features require upgrading to the latest version of iOS. Usually this is because new versions of iOS regularly add new technologies that we'll use to power new features. From time to time, we require an upgrade because an older version of iOS contains serious bugs.
Normally iOS automatically upgrades itself. If you unintentionally turned off auto-updates, it's easy to ask iOS to update itself. Just open the iOS settings app, and tap "General."
Then tap "Software Update."
If there are any updates available, you'll see a button to request an update.