If you run a Ubuntu Server I hope you’re maintaining it up to date (.. and this its true for any OS..). If you dont know how to do so, it just takes a few commands.
First of update your packages :
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
Then update to the latest distribution if there is one :
sudo apt-get install update-manager-core
sudo do-release-upgrade
Et voilà!