I want to upgrade from ubuntu 10.10 to ubuntu 11.04.
The official ubuntu.com page has information:
You can easily upgrade over the network with the following procedure.
- Open the Software Sources application from the System -> Administration menu
Select the sub menu Updates from the Software Sources application:
Change the Release Update drop down to "Normal Releases" and close the application
- Open Update Manager from the System -> Administration menu
- Click the Check button to check for new updates.
- 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.
Follow the on-screen instructions.
Taken from: https://help.ubuntu.com/community/NattyUpgrades
If you're moving from 10.04, see this question:
From 11.04 onwards,when you boot the livecd and start installing it will give a option of upgrading to 11.04. it will automatically detect installed apps and install the updated version of your apps also. assuming your not dual booting