Fully charge the battery if it is not more thatn about 50 percent then turn of the mobile open your pc install maemo flasher then dl the image put it ithe same folder as the flasher slide open the n900 leyboard while turned off press and hold u while connect mobile to pc via usb then open the flasher and type
flasher-3.5 -F <firmware-image> --flash-only=kernel -f -R
type instead of
<firmware-image> the name of the firmware image file
you wont lose any of your settings or apps or anything except kernel power which you can easily reinstall