If you are using Ubuntu 10.04 LTS and were expecting Ubuntu 12.04 LTS to be available in the Upgrade Manager, you were wrong. Well, at least I was wrong…
Canonical won’t make it available via the Upgrade Manager in the first release, but only in July with Ubuntu 12.04.1 release, and the company recommends that most LTS users wait until then before upgrading to 12.04.
But if you can’t possibly wait that long, you can run:
to upgrade from Ubuntu 10.04 (Lucid Lynx) to Ubuntu 12.04 (aka Precise Pangolin). The first command will run in the terminal, the second will start the Update Manager and allow you to select “New Ubuntu release ‘12.04′ is available”.
For details on why you should NOT upgrade yet, see http://askubuntu.com/questions/125825/upgrading-lts-to-lts-server-why-wait-for-the-first-point-release.
The first time I tried to upgrade I had the following errors:
sudo apt-get remove xserver-xorg-video-all
The upgrade process using “do-release-upgrade -d” could continue.