Check whether Apple is still signing the iOS version you want to install
Back up your iPhone or iPad before downgrading
Download the IPSW file for the signed iOS version that matches your device
Put your device into Recovery Mode
Connect your device to a Mac or PC
Open Finder on macOS Catalina or later
Open iTunes or Apple Devices on Windows or older macOS
Select your device
Hold Option on Mac or Shift on Windows and click Restore iPhone or Restore iPad
Choose the downloaded IPSW file
Wait for the restore to complete
Set up your device after the downgrade
Restore your backup only if it was made on the same or older iOS version
If the version is no longer signed, downgrade is not possible through normal restore methods