Step 1. Backup your iPhone to iCloud
Step 2. Reset All Settings
Step 3. Disable “Find My iPhone”
You should turn off “Find My iPhone” before using iTunes to restore or downgrade iOS version. If you forget to turn off this feature, the program will also prompt you to disable this feature when deleting iPhone data and content.
First, go to the Settings app on your iPhone, then click on the device name and check “iCloud” to turn off the “Find My iPhone” option.
Step 4. Full Backup iPhone to Apple (iTunes or Finder)
Step 5. Download the Corresponding IPSW File
Then you need to download the extract IPSW file for the specific iOS version. You can choose the most appropriate file and download it to your computer.Download ISPW
Step 6. Downgrade iOS 16 to iOS 15 with iTunes or Finder