Here you can find all the direct links to the iPhone, iPad, iPod Touch and Apple TV Firmware Files for every released firmware version.
After downloading the firmware, you will need to use the latest iTunes program to restore or update this firmware.
Please note that Apple only signs the current firmware, which mean you can only update the lastest ios version only!
Download instruction: if you use Safari, you must disable the auto unzip feature. It is better to use Chrome or Firefox or download manager to download the firmware link.