Upgrade to Ubuntu 10.10 from Desktop version of Ubuntu 10.04

You want to upgradet to latest Ubuntu version(10.10) and you don't get it in your update menu(upgrade to Ubuntu 10.10), then you need to read the following :

Network Upgrade for Ubuntu Desktops (Recommended) :

1. System -> Administration menu->Software Sources application
2. Open Updates from the Software Sources application window
3. Change the Release Upgrade drop down to "Normal Releases" and close the application
4. Press Alt-F2 and type update-manager
5. 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.
7. A message will appear informing you of the availability of the new release.Click Upgrade.Follow the on-screen instructions.

Source : Ubuntu Community