There are different steps to use, depending on what operating system you have running on your computer.

Windows: Windows users can update their Nokia N900 using Nokia Software Update tool. You can use this walkthrough to install and run NSU under Windows 7, too. Be sure to backup any critical information before applying the update, as it will reset your device to factory settings. The internal device memory will be cleared, but your 32GB internal memory card will not be.

Linux: Linux users can use the Flasher to update their firmware. Download the flasher here, and the N900 firmware image here. To start flashing, turn the N900 off, then connect the USB cable (with the device still powered off). Open a terminal and run ‘sudo ./flasher-3.5 -F <firmware image filename> -f -R‘ and enter your administrator password. You’ll get a message that a suitable USB device isn’t found, so you’ll now need to press the power button to turn your N900 on. Once it’s on, it will start flashing, and then it will reboot. When you see the normal desktop, you can disconnect your N900.

Mac OS X: Follow the same steps as for Linux users, except you’ll need to download the 770Flasher application. As with Linux, you’ll want to connect your N900 (powered down) via USB cable, then run the 770Flasher and drag the firmware image that you downloaded (ends with .bin) over to the 770Flasher. It will tell you a suitable USB device wasn’t found, and that’s when you press the power button to turn the device on. It should then begin flashing your N900, and again, when you see the normal desktop on the device, it’s safe to disconnect.

http://bestof.eblogza.com/2010/02/21...%99s-firmware/