Upgrade to 12.10 from 12.04 not available in update manager

Solution 1:

Open "Update Manager" > click on "Setting" (bottom left) > change "Notify me of a new Ubuntu version" from "for long term support version" to "for any new version" > close

the again "check" for updates.

Solution 2:

Was able to upgrade my system by downloading the Ubuntu 12.10 desktop iso image, writing it to DVD and booting from it. The installation utility has an option to upgrade the current installation rather than reinstalling a complete new system.

I did not lose my home-folder data, however, I lost all other system configurations and had to reinstall all non-default apps. I suggest making backups before installing.