How to recover from iPhone update gone bad

Q. Ever since I upgraded my iPhone to iOS 9.2.1, it’s been acting crazy. It powers off when I don’t want it to, then won’t let me shut if off when I try. How do I shake it out of this?
A. The latest update to Apple’s mobile operating system landed poorly on the iPhone 5s of the friend who sent this e-mail. The symptoms included not just those shutdown snafus but also weird behavior with scrolling and typing that she charitably summed up as “lots of very funky stuff going on.”
To fix that, you should try doing the update the old-fashioned way--through Apple’s iTunes desktop software.

I know, iTunes is nobody’s favorite app from Apple these days. But it also retains the singular ability to perform a complete backup of a phone, down to its saved passwords, and then reload your apps, data and settings intact after restoring the system software to factory condition.
So either fire up your own Mac or PC or that of a friend -- you can also go to an Apple Store -- and plug your iPhone into it. Select the phone in iTunes and click the buttons to set up a backup to “This computer” and to encrypt it, then click “Back Up Now.”
You’ll have to set a password to protect that backup, but OS X will offer to remember that for you. I won’t scold you if you write down that password, as long as the paper involved isn’t a Post-It note stuck to the monitor of a computer other people use.
After the backup finishes, click the “Restore iPhone” button to have iTunes erase the device, install a clean copy of the latest version of iOS and then put back your own apps, data and settings. That should generally work; see, for example, this report on Apple’s tech-support forums of a successful resolution via this route.
If your iPhone has descended into a state in which it won’t even turn on, try its “Recovery Mode.” Plug it into the computer, then press and hold its Sleep/Wake and Home buttons until it restarts and then goes from displaying a white Apple logo to showing the iTunes icon.
At that point, iTunes will open a dialog asking if you’d like to “Update” or “Restore” the device. The first will put a clean copy of iOS onboard while preserving your stuff; the latter will wipe the phone first before reinstalling iOS, so you’d best have a backup handy.
If you can’t even get that to work, you can try putting the device into “DFU” (Device Firmware Update) mode, in which no part of iOS runs and iTunes sees it as a blanked device that can only be restored to factory settings. For help with the somewhat tricky sequence of button pushes that invokes DFU mode, see Rene Ritchie’s walkthrough at iMore.
What if you have an Android device that’s been misbehaving? In Google’s mobile operating system, you can restore a device to stock condition in the Settings app: Tap “Backup & reset,” then tap “Factory Data Reset.”
You can also invoke Android’s own recovery mode if the phone has become too unstable to let you navigate to that part of the Settings app, but the procedure involved varies by phone, so you’ll have to check the documentation for your own model. (Sorry!)
Android and iOS, however, stop resembling each other once you try to get your old data back in place. Although much of your information should be automatically synced to the cloud services of Google and others, a lot of app-specific data is not, which can leave you stuck with some tedious reconstruction of your settings. (Sorry!)