The usual reason for not being able to update the scripts is that your permissions are wrong in your Firefox folder. Your Grease Monkey scripts will be in your firefox folder. The default place for this is (please substitute {username} for your actual Ubuntu user name):
cd /home/{username}/.mozilla/firefox/gm_scripts/
You need to make sure you are the owner of this folder. This command will make sure you are. Again please substitute whats inside the {} brackets.
chown -R {username}:{usergroup} /home/{username}/.mozilla/firefox/gm_scripts
If that still fails to fix the problem, then you also need to make sure you have write permissions on your firefox settings folder.
chmod -R 755 /home/{username}/.mozilla/firefox/gm_scripts
Ubuntu Firefox Linux Permissions Greasemonkey