Check whether Apple is still signing the iOS version you want
Back up your iPhone or iPad
Download the IPSW file for the target iOS version
Put your device into Recovery Mode or DFU Mode
Connect your device to a Mac or PC
Open Finder on macOS Catalina or later, or iTunes on Windows/macOS Mojave and earlier
Select your device
Hold Option on Mac or Shift on Windows and click Restore iPhone/iPad
Choose the downloaded IPSW file
Confirm the restore process
Wait for the downgrade to complete
Set up your device after the restore
Restore your backup only if it is compatible with the downgraded iOS version