Before you get started you will need to download your desire software iPhone Firmwares | Free SmartPhone Unlock Servce iPhone Firmwares . for the list of software.
After downloading the desired firmware below, you can shift-Right Click (For PC Users) or option-click (For Mac User ) the Restore or Update buttons in iTunes. A file dialog Box will open to let you choose the downloaded ipsw-file.Note: Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.

See More: iPhone 3/3gs/4/4s/5/5s/and 5C Firmwares to Update and restore