Sometimes, when you logged in into your server, you have found like the picture below.
Ubuntu tells you that you have 20 packages can be updated and 20 updates are security updates.
It's easy when you have a graphical user interface. You can go to your 'update manager GUI' and just press update.
Well... in this post, I will show you how to upgrading Ubuntu via Terminal
First, you must updating your repository;
sudo apt-get update
And upgrading with this command;
sudo apt-get dist-upgrade
That's it. Easy, right...