Method 1. How to Downgrade iOS 12 to iOS 11 Using IPSW File
Step 1. Download the IPSW file for your device and iOS 11.4 here.
Step 2. Go to "Settings"> "iCloud" and turning off "Find My iPhone" feature on your device.
Step 3. Launch the latest iTunes on your computer and connect your iPhone or iPad to computer with a USB cable.
Step 4. Once iTunes detects your device, please click iPhone icon in iTunes and click "Summary" option.
Step 5. Hold down the Shift key on your PC (or Option key on Mac) and click "Check for Updates" on iTunes.
Step 6. Go to the previously downloaded IPSW file and click "Open" option to install it on your iPhone. After installing the file, you will be downgraded to iOS 11 as you wish.
Step 7. If you have an available iTunes backup, please click "Restore Backup" option and select a backup in iTunes. Make sure it's the correct backup of iOS 11. Please be noted that the latest backup may be a backup of iOS 12, instead of iOS 11's.
Then simply wait till the whole restoring process is complete.
Method 2. How to Delete Update on iPhone via Recovery Mode
Step 1. Launch iTunes on your computer and connect your device to computer with a USB cable. Make sure you have installed the latest iTunes on your computer.
Step 2. Now, please put your device into Recovery Mode with the following instructions.
> On iPhone X/8/8 Plus or later, please press and release the Volume Up button quickly. Then continue to press and release the Volume Down button quickly. Finally, hold down the Side button until you see the Recovery Mode screen.
> On iPhone 7/7 Plus, please press and hold the Sleep/Wake + Volume Down buttons simultaneously. Do not release either button when you see the Apple logo on screen. Keep holding down both buttons until the Recovery Mode screen shows up.
> On iPhone 6s/6s Plus/6 Plus/6 or earlier, iPad or iPod touch, please press and hold the Sleep/Wake + Home buttons simultaneously. Do not release the button when you see the Apple logo on screen. Continue to hold down both buttons until the Recovery Mode screen appears.
Step 3. Click the "Restore" button when it shows up, which will erase data on your device and install the current nonbeta version of iOS on your device, that is iOS 11. Wait for while till the restore process is complete.
Step 4. If asked, please enter your Apple ID and password. This will disable Activation Lock on your device. The restore process won't finish?
Step 5. Once the restore process is complete, you might set up your device with your archived backup. And this will restore backup file to your device.
How To Downgrade iOS 12 to 11.4 on Your iPhone?
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.
Let me you how to downgrade iOS12 to iOS 11.4-
Step By Step Procedure How To Downgrade Your iPhone
Disable 'Find My iPhone'
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.
Make sure you have connect Your iPhone to iTunes
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 iTunes. You'll know you're connected to iTunes when you see the iPhone icon in the top-left corner of the window.
Now set to iPhone into 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.
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.
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Now time to restore your iPhone OS
When your device in recovery mode and connect data cable to your phone 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.
Now select "Restore and Update" on the next popup to start erasing your iPhone and installing it back to factory settings with iOS 11.4. It will downgrade iOS
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."
If your archived backup of iOS 11.4 is encrypted, you'll need to enter your password to continue. Then-
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.
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