Basta digitar esses comandos no console:
source /etc/lsb-release
NEW_RELEASE=trusty
sudo cp /etc/apt/sources.list /etc/apt/sources.list.$DISTRIB_CODENAME
sudo sed -i s/$DISTRIB_CODENAME/$NEW_RELEASE/g /etc/apt/sources.list
(Ele também fará backup da sources.list atual)