First goto http://mvktech.net and then there downloads section. For Nvida go to ulitites and then download the latest version of nvflash. Now depending on what you want to do, there are several different combinations. For basic flash with nvflash back up your current bios with nvflash -b (filename to save as) next to flash the new bios type nvflash -4 -5 -6 (filename of bios)
I would include ATI bios flashing but none of the utilites atm support X850+ gen cards. IF you have questions about older ones feel free to ask. Also if you would like to know how to tweak your timeings I can show you taht also