Roll back to 15.2

Since 15.2 is the latest iOS as far as I know, are you running beta software? If so,

To remove the developer beta immediately, you need to erase and restore your device. Then—if you have an archived backup—you can set up your device again from that backup.

Note that backups created while using beta software might not be compatible with older versions of iOS or iPadOS. If you don't have an older backup that was made using the current version of iOS or iPadOS, you might not be able to restore your device with your most recent backup.

1. Make sure your Mac has the latest version of macOS or the latest version of iTunes.
2. Connect your device to your computer, then put your device in recovery mode with these steps:

• • On an iPad without a Home button: Press and quickly release the volume button closest to the top button. Press and quickly release the volume button farthest from the top button. Press and hold the top button until your iPad begins to restart. If you're not sure, see which buttons you need to press to restart your iPad. Continue holding the top button until your iPad goes into recovery mode.
  • For iPhone 8 or later: Press and quickly release the volume up button. Press and quickly release the volume down button. Then, press and hold the side button until you see the recovery mode screen.
  • For iPhone 7, iPhone 7 Plus, or iPod touch (7th generation): Press and hold the Sleep/Wake and volume down buttons at the same time. Don't release the buttons when you see the Apple logo. Continue to hold both buttons until you see the recovery mode screen.
  • For iPhone 6s and earlier, iPad with a Home button, or iPod touch (6th generation or earlier): Press and hold the Sleep/Wake and Home buttons at the same time. Don't release the buttons when you see the Apple logo. Continue to hold both buttons until you see the recovery mode screen.

1. Click the Restore option when it appears. This erases your device and installs the current non-beta version of iOS or iPadOS. If the download takes more than 15 minutes and your device exits the recovery mode screen, let the download finish, then repeat step 2.
2. Wait for the restore to finish. If asked, enter your Apple ID and password, which disables Activation Lock. If the restore process won't finish, learn what to do.

After the restore finishes, you can set up your device from your archived backup, which must be from an earlier version of iOS or iPadOS.

If you back up a device that has iOS or iPadOS beta to iCloud or your computer, the backup won’t work with earlier versions of iOS or iPadOS. For example, if you go back to iOS 11.4.1 from iOS 12 beta, a backup you made while using iOS beta won't restore. Instead, restore from a backup that you made before you installed iOS or iPadOS beta.

Hello ~ 15.2 is the latest version of iOS. If you are asking about reverting to a lower version of iOS that is not supported.

~Katana-San~