This story is being featured as part of our “Yahoo Best of 2015” series. It was originally published on July 7, 2015.

It’s summertime, and you know what that means: Apple is readying a new version of the software that drives its iPhones, iPads, and iPod touches. It’s called iOS 9, it comes out in September, and it will be free. (It runs on the iPhone 4s, iPad 2, iPad mini, or iPod touch fifth generation — or later models.)

The new iOS version looks and feels just the same as before, and everything’s in the same places; nothing new will confuse you. But don’t let appearances fool you; Apple’s software plastic surgeons have made hundreds of little tweaks that are intended to make its mobile devices faster, smarter, and more stable.

If you want an an early look at iOS 9, you’ll be able to download the public beta version this week (sign up here); just understand that it’s not finished and may be buggy. But I’ve already been playing with the beta and thought you might appreciate a guide to what’s new and useful — including, by the way, a slew of new features that Apple hasn’t announced publicly.

Close the door and cancel your appointments; it’s a very long list. Fortunately, if you squint your eyes a little, you can pretty much fit all of them into six categories: Basics, Apps, Brains, Mail, Camera, and iPad Specials.

The Basics

In iOS 9, Apple has put a lot of work into making things feel faster and more fluid — especially opening and moving between email, messages, Web pages, and PDF files.

The app-switching screen has been redesigned. The “cards” that represent your open apps now overlap, so that more of them fit on a single screen.

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There’s longer battery life for all, too. On average, Apple says every iPhone will get a full hour more of life from every battery charge, which is completely awesome.

It eked out this extra juice by making a long list of tiny tweaks. One example: If your phone is face-down on the table, the screen no longer lights up when you get incoming notifications.

There’s also a new Low Power Mode, clearly inspired by the similar feature on Android phones. You can turn it on at any time, in Settings -> Battery — but you’ll be invited to turn it on when your battery sinks to 20 percent and again when there’s 10 percent left.

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In Low Power mode, the phone stops fetching new mail and updating apps in the background. Most of the cute little animations are eliminated. The processor slows down, meaning that it’ll take longer to, for example, switch between apps. And the battery indicator turns yellow, so you don’t think your phone has suddenly gotten slow just to annoy you.

Apple says that in Low Power Mode your phone or tablet can hobble along for another three hours, which can be a lifesaver.

(By the way: The same Settings screen offers far more information about which of your apps are scarfing down your battery power, as you can see above at right.)

Apple has put some time into beefing up security, too; the most visible example is that you now need a 6-digit Simple Passcode to unlock the phone. Bye-bye, four-digit Simple Passcodes. (You can, of course, still create passwords of any length and style if you turn off Simple Password. And you can still use your fingerprint on recent models.)

Even the upgrading process has been upgraded. Now you need only 1.3 free gigabytes on your phone to perform the OS surgery — not 4.6 gigabytes, as before. If necessary, the phone will even ask if it can delete some of your apps to make some temporary room for the upgrade process. It also promises to put them back at the end.

And the “Upgrade now?” screen offers more choices for the timing — like “Tonight” or “When I use my phone least.”

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