Solution implies reflash phone and for someone could be a hard way, be careful

IMPORTANT, made a backup using backup app before do nothing. Reflashing will delete all the programs you have installed, not the settings of the programs. After reflash you could go to backup app and restore everything.

Be cautious, I deselect all the themes when I was restoring applications.

Easy option if you use Windows use Nokia Software Updater, or not, be careful, but do a backup before to be able to restore applications and some settings after reinstall update.

If not: http://wiki.maemo.org/Updating_the_firmware