Now, an upgrade from Ubuntu 12.10 Quantal Quetzal to Ubuntu 13.04 Raring Ringtail is possible, but it is not recommended because the new Ubuntu 13.04 is an Alpha Testing version.
However, if you want to do this system upgrade, do it at your own risk!
1. The easiest way to update from 12.10 to 13.04, just open the terminal and type this:
$ sudo do-release-upgrade
2. Another method to upgrade from 12.10 to 13.04, harder than the first one listed, is to modify the /etc/apt/sources.list for 13.04 and update the system, like this:
$ sudo sed -i 's/quantal/raring/' /etc/apt/sources.list
$ sudo apt-get update && apt-get dist-upgrade
3. The hardest way is to download the .iso image from here: http://cdimage.ubuntu.com/daily-live/current/, put it on a stick or dvd and install it, by hand.
I recommend using the first method if you really want to test the new Ubuntu Raring Ringtail, because it is the easiest. If you are new to Ubuntu and don’t have the previous 12.10 version, do like I have told you in method 3.