we are also talking about this solution on a french forum, but here the way to find the file is "under subfolders of Documents and settings", not ProgramData, or the simplest is to launch a search.
so upload file here : http://tablets-dev.nokia.com/nokia_N900.php
then have just to replace the file RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin instead the same one find and enjoy

never do it myself, if it doesn't work here's the commands : http://wiki.maemo.org/Updating_the_tablet_firmware