I have Ubuntu 11.10. When I go to Update Manager it does not show 12.04 as available to upgrade system. In settings there is no way fro
Run this command in terminal:
update-manager -d and you'll be ok.
update-manager -d in terminal results in a message like: "system is up to date" then maybe the server you are using to upgrade is not updated. you can change the server in the setttings of update manager in the "ubuntu software" tab; change to main server and try again
update-manager -d, it worked for me this way