You will have to upgrade to Quantal first.
Code:
sudo apt-get install update-manager-core && sudo do-release-upgrade
Source
Raring Ringtail is a develpment release, so the command is slightly different. You need to already be running Quantal.
Code:
sudo apt-get install update-manager-core && sudo do-release-upgrade -d
SOurce, same page as before.