The official ubuntu.com page has information:
You can easily upgrade over the network with the following procedure.
Launch the software center.
Go to Edit -> Software Sources
Select the sub menu Updates from the Software Sources application:
Change the Release Update drop down to "Normal Releases" and close the application
- Launch the update manager.
- 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.
From the technical overview:
To upgrade from Ubuntu 11.04 on a desktop system, press Alt+F2 and type in
update-manager (without the quotes) into the command box. Update Manager should open up and tell you: New distribution release '11.10' is available. Click Upgrade and follow the on-screen instructions.
To upgrade from Ubuntu 11.04 on a server system: install the
update-manager-core package if it is not already installed; launch the upgrade tool with the command
sudo do-release-upgrade and follow the on-screen instructions. Note that the server upgrade is now more robust and will utilize GNU screen and automatically re-attach in case of e.g. dropped connection problems.
Upgrading by using the CD or USB image
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 you're not dual booting.
Upgrading using the command line:
do-release-upgrade in a terminal