Which devices are supported?
Everything that runs iOS 8! That's the fifth-gen iPod touch, the iPhone 4s and newer, all three iPad mini models, and iPads going back to the iPad 2. (The iPad 2!)
Not every feature will be supported on all devices, though. Split View (running two apps side-by-side) works on iPad Air 2 only. And to begin with, you will probably be running Apple apps. (Say, looking up info in Safari while writing a message in Mail or taking notes in Notes.) Third-party developers will need to update their apps to support the feature, but Apple says it's a pretty easy update, akin to the work they had to do to support the different screen sizes of the iPhone 6 and 6 Plus. Still, they might not all scramble to update their apps for the beta, but Apple's apps will work.
The Slide Out and Picture in Picture features for iPad will work with the iPad Air 2, iPad Air, iPad mini 3, and iPad mini 2.
Is there anything special I should do to protect my data while I'm running iOS 9?
Back up and keep backing up. Back up like it's an Olympic sport and you're an alternate trying to make the first team. Back up your photos, and not just your Camera Roll. Someone sends you a cute photo or video in a text? Save it to your Camera Roll so you can back it up. Basically you don't want to have any files that are stored only on your iPhone or iPad running the beta. Everything should exist in another location too.
Since so much of what we do with our iPhones is cloud-based, you might not keep a ton of documents stored locally on your iPhone, and it can be easy to forget to back up. So give it some thought. Look at the apps you use--do they store things in your Dropbox, in your iCloud Drive, or in another cloud location? You should be OK. But files you create on your device--photos and videos spring to mind, but there might be more--needs to be backed up often.
How do I install updates to the beta?
Apple will update the iOS 9 public beta a few times before the final version ships. But updating it is easy--you can update over the air in the Settings app, or use iTunes. It's always a good idea to back up before updating, of course.
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