When trying to upgrade from Ubuntu 12.04 to 12.10 through the Update Manager, it says there are no updates to install.
Make sure you have configured your Update Manager software to show upgrades to non-LTS versions. (By default, it will only display alerts when an upgrade to a new LTS version is available.)
Mike Stewart wrote a visual guide in another thread: "Upgrading doesn't work on my laptop for 12.04?" Take a moment to review if this was the cause of your problem. :)