To upgrade from Ubuntu 12.04 LTS on a server system: ensure that
vim /etc/update-manager/release-upgrades includes Prompt=Normal.
Install the update-manager-core package if it is not already installed; launch the upgrade tool with the command
sudo do-release-upgrade -d
and follow the on-screen instructions. Note that the server upgrade will utilize GNU screen and automatically re-attach in case of e.g. dropped connection problems.
Offline upgrade options via alternate CDs are no longer offered for Ubuntu Desktop and Ubuntu Server. Please ensure you have network connectivity to one of the official mirrors or to a locally accessible mirror and follow the instructions above.