Here are the steps provided by Nikon - I'll add my comments in CAPS.
Basic Upgrade instructions: for detailed instructions, see pdf files below.
YOU MUST START THE UPDATE WITH A FRESH OR NEARLY FRESH BATTERY. IT IS EXTREMELY IMPORTANT TO HAVE A STRONG ENOUGH BATTERY THAT THE CAMERA DOES NOT SHUT DOWN FOR ANY REASON UNTIL THE FIRMWARE INSTALLATION IS COMPLETE.
1. Download and expand the firmware files - THE FILES ARE ZIPPED AND JUST NEED TO BE EXTRACTED 2. Format an approved CF card in the camera (do not use a Microdrive card to perform this update) 3. Connect Camera to computer (in MSC USB mode) or use a CF Card Reader (do not use a Lexar Jump Shot USB cable to perform this update) 4. Copy to the top level of the card the downloaded "A firmware" file COPY IT TO THE ROOT - DO NOT PUT IT IN A FOLDER. THE CAMERA KNOWS WHAT TO DO AND WILL NOT PROCESS THE FIRMWARE IF HANDLED INCORRECTLY. 5. Disconnect camera from computer 6. Update the firmware from the "Setup" menu - THIS WORKS PRETTY SEAMLESSLY - JUST SELECT THE A FIRMWARE YOU WANT TO UPDATE AND FOLLOW INSTRUCTIONS. BUT YOU ONLY INSTALL ONE FIRMWARE VERSION AT A TIME. 7. Follow the camera's on-screen menus 8. Repeat steps 2 through 7 using the "B Firmware" HAVING DONE THIS ONCE, IT IS MUCH EASIER THE SECOND TIME FOR THE B FIRMWARE 9. Re-format Card
After completion, check the firmware and make sure both A and B are updated.
The good news for newer cameras is they have combined the firmware updates into a single update without having to separately update A and B.