Following instructions from here: http://wiki.maemo.org/Updating_the_tablet_firmware

In Simple form:

Flashing using the command line
In order to flash directly from the XP/Vista/Windows 7 command line, follow these steps:

1. Download and install the latest version of Flasher e.g. maemo_flasher-3.5_2.5.2.2.exe from the website above.
2. Download the latest firmware (.bin) file and save it to %ProgramFiles%\maemo\flasher-3.5\ (the default installation path for Flasher) or to the custom path of your choice.
3. Ensure the battery is at least half full.
4. Unplug charger cable and switch off the device.
5. Open the Command Prompt (Start then Run or Windows Logo key + R) and type cmd then press Enter.
6. Change the directory to the Flasher's path (TIP: Use the TAB key to auto-complete the commands and file-names)

cd "%ProgramFiles%\maemo\flasher-3.5"

7. Hold the u key on the device keyboard and connect the USB cable at the same time (see the Flasher documentation)
8. Run the following command:

flasher-3.5.exe -F RX-51_2009SE_10.2010.19-1.002_PR_COMBINED_002_ARM.bin -f -R


The update will take approximately 4 minutes after which the N900 will reboot and display the Welcome Screen with Regional settings.