If you've already been using iMazing to back up your iOS devices, great! Scroll through your backups to locate one that was made on the iOS version (in this case, iOS 12.4.1) that you want to restore, If you've got one, super! If not, but you have one on your Mac that was made by iTunes before you upgraded to the version you want to avoid, you may still be OK. Honestly, I've never used the iCloud iOS backup option, so I don't know if it will work, but it's worth a shot.
Worst case, with no device backup for the earlier version, you'll have to set up your iPhone / iPad as if it were brand new, manually download your apps, fix your settings so they suit you, etc. But—you'll have your old iOS version back! And if you can restore from a version before the problematic system upgrade, you're golden. If you have more than one iOS device to revert, I suggest you revert one at a time all the way to configuration complete, A working iOS device will make your reversion easier—you can copy some settings from it after reversion automatically, and you'll have an example of how you like to set up your devices if you have to restore manually.
Be sure to backup any data that doesn't reside in a cloud service so you can restore it later!
- Install Imazing if you haven't already.
- Download the .ipsw image file for the iOS version you want to revert to from IPSW Downloads.
- You'll need your "Axxxx" model number for your iOS device. You can find it either on the back of your iOS device, or under Settings->General->About. If the Settings app shows a model number that begins with an M, just tap it and the model number beginning with A should be shown.
- Click on the "Identify My Device" link on the IPSW.me site, select your device type (iPhone, iPad, etc.) and enter the Axxxx model number.
- Select the version you want to download, and then click the download button. WARNING: the site is heavily ad-supported. Be sure to click the real download link! it's inside a blue box and will have the download size in parentheses.
- Connect the device you're going to revert to your Mac with a USB cable.
- Launch iMazing.
- You''ll see that iMazing has four panels. Select the device you want to revert from the left-hand panel. It should appear in the right-hand panel.
- Scroll down in the third panel, and click on "Reinstall iOS / iPadOS".
- Tap on the "Restore from .ipsw" button and select your downloaded ,ipsd file
- Be sure to check the "Erase all data" box. My experience is that the installation won't work if you don't.
- You'll get a warning dialog as in Figure 1 below. Go ahead and click Continue, but heed the warning about iTunes! If iTunes pops up (and it did for me, twice), click "Cancel" in its dialog box, and quit it, as in Figure 2 below.
- Eventually, you will get the set of screens on your iOS device that you get on a brand new out-of-the-box device! Ignore the offer to restore for now, and get to the Home screen as quickly as you can. But be sure to NOT turn on automatic updates!
- Your should now have the iOS / iPadOS verson you want! If you have a backup from before your ill-considered upgrade, go ahead and use it to restore now in whatever backup software you have. Otherwise, go about updating your settings, installing apps, etc. to bring your device back to life. Again, check to be sure you've turned off automatic updates! This is at the very bottom of the Settings app->General->Software Update panel. Triple check this!
- Ignore all of Apple's future pernicious attempts to get you to upgrade before you're ready.