I am trying to update to Ubuntu 12.10 from 12.04, but the update manager never gives the option, even if I have selected 'Notify me of a new Ubuntu version: For any new version'.
This is how it should work(tested myself):
That should force the check for Ubuntu 12.10. If that still doesn't work you can do the less pretty way and use terminal.
This method is usually used for Servers since they lack a GUI but it works for Desktop releases too.
Just open a Terminal and enter these commands:
That should do it. Although I always recommend making a Backup of any important data. I actually go the extra Mile and Image my entire HDD using