do-release-upgrade utility is used to upgrade to the latest version of Ubuntu server.
To collect latest package information, execute update apt-get
sudo apt-get update
You have to install the upgrade utility then.
sudo apt-get install update-manager-core
And then, finally you run do-release-upgrade
Use the apt-get tool
sudo apt-get upgrade
To automate package updates, use unattendedupgrades:
Install the package first:
sudo apt-get install unattended-upgrades
You need to configure it then:
sudo vi /etc/apt/apt.conf.d/50unattended-upgrades
Unattended upgrade actions are logged in /var/log/unattended-upgrades. Add a Cron job to run /usr/bin/unattended-upgrade periodically.