If the update doesn't show up for you at all, you may have accidentally (or intentionally) uninstalled osso-software-version-rx*4, which is required to update. You can get it back by simply running an apt-get install osso-software-version-rx34, for the N800, and an apt-get install osso-software-version-rx44, for the N810. Then updating your repository list in Application manager.