This step will guide you to upgrade your Ubuntu Server 14.10 to Ubuntu 15.04:
Install the update-manager-core package if it is not already installed.
sudo apt-get install update-manager-core
Make sure the /etc/update-manager/release-upgrades is set to normal.
Launch the upgrade tool with the command sudo do-release-upgrade.
Follow the on-screen instructions. Usually you can type “y” for all question provided.