iOS: For most iPhone users, updating to iOS 12 should be smooth sailing: Faster speeds and fun new features for all. But while we have yet to experience any issues with iOS 12 on our devices here at Lifehacker, we have seen a number of reports from others — especially those with older devices — who feel as though they’re having battery issues after updating to Apple’s latest version of iOS.
If you’ve experienced issues with iOS 12 and they’re enough to make you run back to the safety of iOS 11, you have a little bit of time to downgrade your device before Apple stops signing iOS 11. (Once that happens, you’re stuck with 12.)
To get started, visit ipsw.me and look up your device. You’ll quickly be able to see whether any IPSW files — iOS firmware — are available for iOS 11.4.1:
If you’re lucky, you’ll be able to download the iOS 11.4.1 IPSW file to a desktop or laptop. Then plug in your iPhone, select it in iTunes, and hold down Shift or Option when clicking on the Restore button. Go find the IPSW file you downloaded, prepare to kiss all of your data and apps goodbye (anything you haven’t synchronised to iCloud, that is), and run the restore.
Remember, since you upgraded to iOS 12, you won’t be able to “restore” your iPhone from a backup that was made in iOS 12. You’ll have to set up your device from scratch, which means tackling a bit of configuration to get your iPhone looking and working just the way it was pre-iOS 12. If you kept a backup of your iOS 11 device before upgrading to iOS 12, you’re golden.
How to remove iOS 12 and downgrade to iOS 11 | Macworld