Apple's goal with iOS 12 is to smooth out the problems with its famously flawed predecessor. Of course, change isn't for everyone, and by iOS 11.4, Apple had a pretty stable thing going. If you're not digging all the bugs and new features included in the iOS 12 beta, here's how to downgrade your iPhone back to iOS 11.4.
As long as you're running the iOS 12 developer or public beta, you'll be able to go back to the last version of iOS 11 using iTunes for macOS or Windows, as long as you archived an iOS 11 backup before updating to iOS 12. However, you can still downgrade to iOS 11 without a backup, only you will have to start with a clean slate.
Once iOS 12 is released to the masses in September, Apple usually stops signing the previous version the following month, which means you might not be able to downgrade then unless you've saved your SHSH2 blobs.
As with any restore, you'll want to disable Find My iPhone, though this can be done later when erasing the device, since you'll be prompted to do so. It's just easier to get it out of the way first so there are no hiccups later.
First, open the Settings app, then tap your name at the top of the list and select "iCloud." On the following page, scroll down and tap "Find My iPhone," then disable "Find My iPhone." Enter your Apple ID password when prompted, and Find My iPhone will be disabled.
Next, connect your iPhone to your computer, then launch iTunes if it doesn't do so by itself. You'll want to make sure you're running the latest iTunes version when doing this, so update first if necessary. You'll know you're connected to iTunes when you see the iPhone icon in the top-left corner of the window.
Before you can go ahead and restore your iPhone back to iOS 11.4, you need to put it in recovery mode. This setting allows your iPhone to restore itself to an official build of iOS. Because different iPhones have different button layouts, however, the method for accessing recovery mode differs from iPhone to iPhone.
With your iPhone connected to your computer still, perform one of the following:
- iPhone X, 8, or 8 Plus: First, press and release the Volume Up button. Second, press and release the Volume Down button. Third, press and hold the Side button until the "iTunes" screen appears.
- iPhone 7 or 7 Plus: Press and hold the Sleep/Wake and Volume Down buttons at the same time until the "iTunes" screen appears.
- iPhone 6S or older: Press and hold both the Home and the Sleep/Wake buttons at the same time until the "iTunes" screen appears.
Once the recovery mode feature is on, iTunes should automatically prompt you that "There is a problem with the iPhone that requires it to be updated or restored." Simply hit "Restore" on this popup. If you hit "Cancel" instead, you can still hit "Restore iPhone" button on the iPhone Recovery Screen in iTunes to continue.
Next, select "Restore and Update" on the next popup to start erasing your iPhone and installing it back to factory settings with iOS 11.4. (You can restore to your archived backup in the next step.)
If prompted to, enter your Apple ID and password to disable Activation Lock, but this shouldn't appear if you completed Step 1 above. Once you see that white welcome screen on your iPhone, congratulations, you've successfully restored your iPhone to iOS 11.4! But you're not done yet.
Since your iPhone has essentially erased itself, you'll need to run through the initial setup steps. If you want to start from scratch, select "Set Up as New iPhone," but if you want all of your apps and data back, tap "Restore from iTunes Backup."
When restoring with your archived backup, make sure you're still connected to iTunes and select your iPhone in the application. From there, select "Restore Backup," then choose your archived backup file from the list, and hit "Restore." (An archived backup will have the data and time in the actual backup name, as opposed to other backups that will just have the device name only.)
If your archived backup of iOS 11.4 is encrypted, you'll need to enter your password to continue.
Wait for the restore to complete before disconnecting your iPhone. It should restart, and it will need to sync with iTunes. Once it's done syncing, you have successfully downgraded to iOS 11.4 with all your data intact.