Now that iOS 9 is being pushed out to a device near you this afternoon, here’s a great article from MacWorld about how to upgrade as painlessly as possible. And some advice on how to downgrade as well, if it’s not working out for you. (Hint: you’ll have to plan for the ability to downgrade before you do the upgrade.)
How to upgrade to iOS 9 (and, if you need to, reinstall iOS 8)