Why do you want to flash the bios? Generally you do not want to mess with bioses unless you have a serious issue, if it is working fine you generally want to leave it alone.
There's a collection of GTX680 BIOSes over here: http://www.techpowerup.com/vgabios/inde ... &memSize=0 and you can flash them using tools like this one: http://www.techpowerup.com/downloads/21 ... ndows.html using this guide: http://www.techpowerup.com/forums/showt ... p?t=119955 but I really don't see the point - my stock GTX680 is fast enough for just about every game that I play, using max in-game settings... there are some games that have poor performance, but they are just poorly optimized and a few extra FPS from overclocking will not do anything useful for these. Plus you always have a chance of ruining your card (unless it has something like dual BIOS setup) when re-flashing it with different BIOS