Apple has been running OS X public betas for both Yosemite and El Capitan, but it only began offering public betas for iOS in March, and those were only for smallish point updates. Now the company is taking a bigger step: it just released the first public beta of iOS 9, the next major release of the company's mobile operating system due out this fall. iOS 9 is mostly focused on polishing up the big additions in iOS 7 and iOS 8, but there are still new things for people to play with. It refines the iOS 7-era UI with the new Apple-designed San Francisco typeface. It includes a more versatile version of Siri that responds better to natural phrases like "show me pictures from last week" and can present information more proactively. The Notes app is better at adding and organizing links, photos, and other things, Maps picks up public transit directions in select cities, and a new News app lets users get a specially formatted, customizable stream of news from a variety of publications. iPhones also get a new Low Power Mode that turns off some background services to prolong battery life in a pinch. iOS 9's biggest changes are reserved for newer iPads, which get some long-rumored multitasking features that make better use of their larger screens. The iPad Air 2 is the only tablet that gets everything—it can run two apps side-by-side in Split View mode, something we examined in greater detail last month. In addition to the iPad Air 2, the iPad Air, iPad Mini 2, and iPad Mini 3 can all use Slide Over, a sort of temporary multitasking that lets you slide an app in from the right side of the screen and work with it for a moment before returning to your main app. And all four of those tablets can also use a new "picture-in-picture" video mode that lets you watch a video as you do things in other apps.
All iPads pick up support for new hardware keyboard shortcuts too, many of which are the same ones used in OS X. And the software keyboard gets some quick shortcut buttons for copying, pasting, and undoing, as well as a new feature where holding down with two fingers lets you use the keyboard area as a trackpad that will move your cursor around.
You can sign up for the public beta here. You'll be given a configuration profile to install, which will enable your phone or tablet to grab the iOS 9 OTA just as it would see a standard iOS 8 update. New public beta builds should follow every few weeks, and you should be able to upgrade from the iOS 9 beta directly to the "Golden Master" build of the OS on release day without wiping your device.
We'll be publishing a broad overview of iOS 9's new features soon. A more in-depth piece, including an extensive examination of performance and battery life, will follow when the final version comes out in the fall.
Beta software is called beta software because it's not finished—it's inherently buggier and crashier than the stable versions of the operating system, and installing it on your phone or tablet could result in anything from reduced battery life to lower performance to data loss.
Install at your own risk and remember, if you have to ask "should I install this" you probably shouldn't.
For those of you not scared away by that warning, some further advice: Apple suggests that you install the iOS 9 beta on secondary iDevices or those set aside specifically for testing. Since iOS 9 supports everything going back to the iPad 2 and iPhone 4S, there's a chance you have a decent test device sitting in a drawer or closet somewhere even though hardware that old won't support all of the operating system's new features.
But what if you don't have an older device, or if you'd like to experience iOS 9 in all its glory on your newest hardware? The good news is that you can always revert to iOS 8 if things go poorly, you just need to make an extra backup first.
Assuming you use iCloud for normal device backups, before you upgrade to the iOS 9 beta you should hook your device up to iTunes and create a full backup. If you check the "encrypt backup" box, it will back your account passwords up for you too. iCloud backups created with newer versions of iOS can't be restored to devices running older versions of iOS—as soon as your iOS 9 device backs up to iCloud, you'll only be able to restore it to other iOS 9 devices. Having an iOS 8 backup sitting in iTunes is a good way to get most of your stuff back.
Apple has instructions for rolling back to the current version of iOS on its support page. You'll need to put the device into recovery mode by powering it down and then holding the Home button down as you plug it into the computer. Restoring the device will download and install a large-ish iOS 8.4 file, which will wipe out any user data on the device. When it's finished, you can restore from the local backup you made earlier. This process completely downgrades everything from the core OS to your modem firmware, and it shouldn't cause any problems for you as long as you've taken steps to preserve your data.