There are several ways to flash your bios from the xbox, the two i use Evolution-X and Flash-X, both are obtained from the usual source as well as the bios, if you want the most up to date bios then get a copy of the xecuter 4983 bios use the 4983.06 if you have a stock hard drive or the 4983.67 if you have a hard drive larger than 137 gigs.
To Flash the bios using evo-x there should be a bios file in the root of your evolution-x folder, for example if your evo-x das is installed on C:\\evolutionx\\ then you want to make a new folder called bios inside there. You then use the flash bios tool inside the settings menu of evo-x you may have to enable this by deleting the "#" from in front of the function in you evox.ini file.
To flash the bios using Flash-x install that as an app you can launch, then make a folder called bios on any either your C:, E: or F: drives, when you launch flash-x you will have to look for the bios ( you'll see what i mean when you load it), then just open your bios select the banks you want to flash and prees Y then X.
Hope that helps you.