To upgrade from Ubuntu server system to the latest server:
1- install the update-manager-core package if it is not already installed:
sudo apt-get install update-manager-core
2- This is optional, you can update without doing this: Edit /etc/update-manager/release-upgrades and set Prompt=normal;
3- Launch the upgrade tool with the command
sudo do-release-upgrade -d
and follow the on-screen instructions.