I am using Ubuntu 10.10, for which the official support period is over. When running the upgrade manager, I also cannot see the option to upgrade to a newer version, are you aware of any alternatives for upgrading.
Make sure that the Update Manager notifies you for any new version and not just for LTS versions.
You can do this by going to the settings (found at the bottom of the page) and in the "Notify me of a new Ubuntu Version" choose "For Any New Version"
There are many ways to upgrade, these steps found at https://help.ubuntu.com/community/NattyUpgrades
1- Open the Update Manager application from the System → Administration menu.
2- In Update Manager, click the Settings... button, and enter your password to start the Software Sources application.
3- Select the sub menu Updates from the Software Sources application.
4- Check the "Release upgrade - Show new distribution releases" drop down to make sure "Normal releases" is selected, and change it if otherwise. Close the Software Sources application and return to Update Manager.
5- In Update Manager, click the Check button to check for new updates.
6- If there are any updates to install, use the Install Updates button to install them, and press Check again after that is complete. A message will appear informing you of the availability of the new release.
7- Click Upgrade.
8- Follow the on-screen instructions.
This link also lists a way to upgade from an alternate cd, https://help.ubuntu.com/community/NattyUpgrades
1- Download the alternate installation CD from http://releases.ubuntu.com/natty/
2-Burn the ISO to a CD and insert it into the CD-ROM drive of the computer to be upgraded.
3- If the ISO file is on the computer to be upgraded, you could avoid wasting a CD by mounting the ISO as a drive with a command like:
4- A dialog will be displayed offering you the opportunity to upgrade using that CD. Follow the on-screen instructions.
5- If the upgrade dialog is not displayed for any reason, you may also run the following command using Alt+F2:
Also there is this way, from http://www.unixmen.com/how-to-upgrade-from-ubuntu-1010-to-ubuntu-1104-natty-desktop-a-server/
1- Press Alt+F2 and type in
2- Now follow these steps : First check for updates, then Install them, and finally click Upgrade and follow the on-screen instructions.
One option you have is to back up your data and do a fresh install of the newest ubuntu os and then restore your data