and install itunes latest version on your PC get it from http://www.apple.com/itunes/download
after install iTunes just connect your phone to PC with the original USB cable.
iTunes automatically detect your phone and it will ask to download and install the latest version firmware to your phone.
if you have good internet connection then at least 45 minutes it will take to recover your phone.
while itunes downloading firmware dond do anything on your phone. just leave it