Ubuntu 16.04 LTS Xenial Xerus: Later today April 21, 2016, the Canonical will release the sixth LTS (Long Term Support) version of its Ubuntu Linux operating system. Ubuntu 16.04 is coming and you better prepare. 
Ubuntu 16.04 LTS is a great traffic with many new features and improvements.
Follow the next instructions carefully for a successful process upgrades.
Before anything else, make sure you have taken a recent backup of the most important ones files you.
To begin the upgrade process, open the "Software & Updates" Unity Dash and go to the "Updates" tab. From there select "For each new version" from the drop down menu "Notify me of a new version of Ubuntu."
Close the “Software & Updates” window, press the two keys Alt + F2 together and type “Update-manager” (χωρίς εισαγωγικά) στο πλαίσιο εντολών.
The Update Manager utility will pop up and start looking for the new updates. When you discover them, it will prompt you to upgrade your system with a window that says
"New distribution release 16.04 LTS' is available." or “The new distribution 16.04 is available”
Click the "Upgrade" button and follow the instructions that will appear on your screen.
____________________________________
If you want to upgrade from the terminal without a graphical interface, follow the steps below after making the changes in "Software & Updates".
Open the Terminal and type the following commands
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
Then to run the software updater run the following command:
sudo update-manager -d
and follow the instructions
Enjoy the new Ubuntu 16.04 LTS.
