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.