Por favor, tente
echo "deb http://archive.ubuntu.com/ubuntu trusty main universe restricted multiverse | sudo tee /etc/apt/sources.list
echo "deb http://archive.ubuntu.com/ubuntu trusty-updates main universe restricted multiverse" | sudo tee -a /etc/apt/sources.list"
echo "deb http://archive.ubuntu.com/ubuntu trusty-security main universe restricted multiverse" | sudo tee -a /etc/apt/sources.list"
echo "deb http://archive.ubuntu.com/ubuntu trusty-backports main universe restricted multiverse" | sudo tee -a /etc/apt/sources.list"
sudo apt-get update
sudo apt-get upgrade
As primeiras linhas redefinirão seu repositório. Então a atualização pode funcionar.