I was updating to Ubuntu 12.04 and I lost my wireless connection. I turned off my computer and got to a better connection and now my "Update Manager" does not prompt for the upgrade.
Make sure that all possible updates are installed using update-manager.
After that you can trigger the update-manager with the dist-upgrade option which should start it showing the upgrade option to 12.04. Do it like this:
If that doesn't work it might be that all your repositories are already changed to the new distribution. You just need to finalize the installation:
Please let us know if one of these ways worked for you.
Uh, good luck; this usually works, but not every time.