Mình sưu tầm được bài viết dưới đây. Bạn tham khảo nhé.
I just downloaded the firmware then pressed "Control" + Upgrade in iTunes (on a Mac). My IP address for "Apple Server" may be changed to Saurik's server, but I'm not positive that I did that. So if this doesn't work for you, try changing the address of the Apple Contact Server to Saurik's server (follow these directions:
Caching Apple's Signature Server - Jay Freeman (saurik)).
Remember to keep your 3.x SHSH on file in case you need to downgrade.
Good luck!