How to update

I am using iPhone 4 GSM. It is running with iOS 6.0.1. I want to update my iOS to iOS 7.0. But in my phone setting> general> software update, there is a notification to update to iOS 7.1.1. I downloaded the firmware of iOS 7.0 for iPhone 4 GSM from your website and I updated my phone to iOS 7.0 with my iTune. It cannont be updated there was an error message while extracting firmware. The error message is " Firmware file was corrupt. Disconnect and connect try again". I don't understand this error. I try to update for three time but the same error was shown. Please tell me what I should do if you can. I am not a jailbreak user. I am using iTune 11.2 version.