I created a USB startup disk from ISO and booted using that to upgrade my 11.10 installation. But when I am not able to select Upgrade from Ubuntu 11.10 to Ubuntu 12.04. The radio button cannot be selected. I am doing a Network upgrade now.
If are trying that upgrade with a "live USB" then you can't. Though if you have the alternate iso, then you could avoid wasting a CD by mounting the ISO as a drive with a command like:
Then to start the upgrade enter: