Howto:
https://www.redmondpie.com/how-to-downgrade-ios-10-beta-to-ios-9.x-tutorial/
Firmware to any iPhone device:
https://www.iphonehacks.com/download-iphone-ios-firmware
Firmware list:
https://4pda.ru/forum/index.php?showtopic=164173
Recovery mode:
– turn off iPhone
– push and hold Home button
– connect iPhone to PC using USB cable
– start iTunes
– recover iPhone: wait until iTunes donload firmware and install it
https://4pda.ru/forum/index.php?showtopic=636263
Flash old or other firmware then currently instlled on iPhone:
– enter DFU mode:
-turn off iPhone
– push and hold Home button
– push and hold Power button
– hold both buttons for 10 sec
– release Power button and keep holding Home button for 15 sec
– connect iPhone to PC using USB cable
– start iTunes
– hold shift button on PC’s keyboard and click on Recover button in iTunes
– browse and select donloaded firmware file
https://4pda.ru/forum/index.php?showtopic=132446
https://4pda.ru/forum/index.php?showtopic=154216
Полезный сайт по софту и железу iPhone
https://www.mobile-files.com/