This question already has an answer here:
- Can I skip over releases when upgrading? 15 answers
My update manager on 12.04 will not find the upgrade to 13.04. I would like to know why I can't see it.
The only possible direct upgrade that is currently available by default for Ubuntu 12.04 is Ubuntu 12.10, which will only be supported by updates until April, 2014. If you wait until April 2014, you can also upgrade Ubuntu 12.04 to the next Long Term Support (LTS) release, which is Ubuntu 14.04 Trusty Tahr. Ubuntu 14.04 will be supported by updates until April, 2019.
If you don't want go through all the extra effort of skipping over the default upgrade release when upgrading just to install a release that will only be supported by updates for another three months until January, 2014, it would be a good idea to wait until April, 2014 and then upgrade to Ubuntu 14.04 Trusty Tahr.
Reference: Can I skip over releases when upgrading?