, please note that Apple stopped signing this specific version on March 31, 2020. This means you can no longer officially restore or downgrade your device to iOS 13.3.1 using iTunes or Finder.
: Offers a list of IPSW links for all compatible devices from its release in January 2020. Download iOS 13.3.1 for iPhone 6s model iPhone8...
: Hosts a repository of Apple firmware links for various iOS versions, including legacy releases. Important Compatibility Notes iOS 13.3.1 (17D50) for iPhone 6s - IPSW Downloads , please note that Apple stopped signing this