You can give a try to Ubuntu Live CD. / Bootable USB
Burn Ubuntu iso to a DVD / Bootable USB and boot from by choosing the option try Ubuntu without install.
Now when you boot yo can mount your harddisk and chroot.
sudo chroot /path-to-mounting-point-of-your-disk
Now do these commands
sudo apt-get update sudo apt-get dist-upgrade
When finish restart and remove the DVD / Bootable USB and you can restore your OLD OS.