It will not show the upgrade, because you probably have set to only notify on upgrades of new LTS releases. This would be 14.04 released next year. If you want to upgrade to a regular non-LTS release like 13.04 is, then you should change that setting.
The command-line way of changing this is as follows:
/etc/update-manager/release-upgrades in your favourite text editor, e.g.:
sudo gedit /etc/update-manager/release-upgrades
Change the line
Save and close.
Run the regular upgrade procedure: