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.