But take the decision carefully, because you probably won't be able to go back
See where your iPhone or iPad sits on the chart of compatible devices above. If it's only just new/powerful enough to run the new OS, you need to find out if there will be any speed problems. See how people with the same model as you get on.
How to update to iOS 9: Important steps to prepare your iPhone or iPad for the upgrade
We're about to walk you through the update process. But before we get to that, here are the steps you should take before you click yes on that update alert. (My colleagues look at a couple of aspects of this in a little more depth in a separate article: How to prepare your devices for iOS 9.)
1) Back up your data
Use iCloud or iTunes to back up your iPhone, iPad or iPod touch. If you don't do this, and you find that messages or photos disappear from your iPhone after the update, then that's your look-out.
Beware: the backup will include purchased music, TV shows, apps, and books; photos and video in the Camera Roll; and device settings, but it won't include anything you synced from your computer. To re-sync that stuff you'll need to sync with iTunes. For that reason we recommend backing up to your computer as well as iCloud.
2) Save a compatible copy of iOS 8 in case you change your mind
Update 18 March 2016: Due to Apple no longer signing off iOS 9.2, you will not be able to downgrade after you update to iOS 9.2 or above (such as 9.2.1)
You may change your mind and decide you want to downgrade from iOS 9 to iOS 8, and this will be easier if you make sure you've got a copy of the latest version of iOS 8 compatible with your device.
You see, Apple 'signs' versions of iOS, which in effect tells your iPhone or iPad that the version you want to use is okay for that device. As long as that version is signed, you can install it on your iOS device, even if it's earlier than the one on there at the moment. (Traditionally, Apple stops signing old versions of iOS only a day or two after releasing major updates, so you'll need to act quickly if you want to go back.)
If you have a copy on your hard drive you will find it, by default, by following this path:youruserfolder/Library/iTunes and then select the Software Updates folder for your device. (Access the Library folder in your user folder by holding down the Option key in Finder and selecting Go > Library.)
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