Force upgrade 12.10 to 13.04
To upgrade from 12.10 to 13.04, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo apt-get update && sudo apt-get dist-upgrade
sudo update-manager -d
Once that is done, a window will open, just click on Upgrade.
Hit Ctrl+Alt+T to open a terminal, and type the following:
update-manager -d