Are you looking for a Step by Step guide ?
I am not sure if you will find one right here ! Btw you can use search button to search for it !
You can do this:
1) Go to sammobile.com, click on "firmaware", search for your phone on the right search box.
2) Download the Official Firmware. When download finished, extrat it somewhere.
3) Download ODIN
4) Install Samsung KIES (it includes USB Drivers) if Kies is already reinstalled, then uninstalled, reboot and reinstall back.
5) Go to download Mode (When powered off: press Volume UP, Home and then Power button and finally Volume Up again)
6) Flash the firmware.
Note: You may change USB Cables, USB Ports (of your Computer) or even Computer if Odin won't detect your Phone !
Alternative Way: If the Phone is switching on, then Download This great app !
When installed, just open it and will installed a flash kernel automatically. After that, just click "Open Tar/MD5/ZIP" then look for your firmware and then click "Flash Firmware".