No facility exists in Update Manager to upgrade to a new version, why?

Because Ubuntu 12.04 is a Long Term Support release (meaning that it is supported with bug fixes and security updates for 5 years) you won’t find a pop-up telling you that a new version is available.

If you want to upgrade,you can follow these instructions

1. Open Update Manager from either the Power Menu or the Dash.
2. Click the ‘Settings’ button in the lower-left corner of the Update Manager window: This will open Software Sources.
3. Click the menu next to “Notify me of a new Ubuntu version” and select ‘For Any New Version’

4. Important DO NOT uncheck or alter anything other than the above.

Once you’ve made the change you will likely be prompted to input your user password. After this, close Software Sources, then close and then reopen Update Manager.

. Click the ‘Upgrade’ button to begin your upgrade to Ubuntu 12.10.

source: OMG Ubuntu

You can also force it to check for development versions (since Quantal is released this won't really make any significant difference for you) by opening a terminal and typing:

update-manager -d &

open Software Sources application (you can find it in the Dock) and go to the Updates tab

You should choose the option For any new version

Then press [Alt] + [F2] and type update-manager -d

Update Manager should offer you an upgrade to the new version.

Run this command in the terminal:

sudo do-release-upgrade

That will do the same as running it GUI :)