Firstly what you need is a Kepler based graphic card for eg:- GTX690, GTX680, GTX670, GTX660Ti, GTX660OEM and GTX660.
Then softwares needed for Editing/Flashing bios are:-
For Unlocking/Editing kepler bios use KGB(Kepler Golden BIOS) :- Download it from here Bios Editor
For Flashing bios use Firestorm :- Download it from here Firestorm
Now steps required to perform:-
1.) Download the BIOS Editor.
2.) Use GPUZ to extract your VGA bios, and make a copy into the KGB folder, and a backup elsewhere.
3.) Hold Shift + Right Click in the KGB folder.
4.) Select "Open Command Window Here"
5.) In the command promp, type "kbg.exe [Your Saved Bios Name].rom unlock"
6.) Download FireStorm and install it. (The install screens are in another language, just keep clicking where the "Yes/Accept" button normally is.
7.) Run Firestorm, click on the "BIOS" button, locate your modded bios that is in the KGB folder, and flash it.
8.) Restart, enjoy!
Between here is the video Guide GUIDE
I tried unlocking bios of my Zotac gtx 670 Amp edition, successfully unlocks it and able to get 40-50 mhz more overclock headroom on core clock .
If for some reason something goes wrong, you can boot off a second gpu(ex iGPU or another dedicated) and reflash the bios you backed up.
Note:- Do at your own risk! I am NOT responsible if anything goes wrong!