Ubuntu upgrade from 14.04 to 16.04
To upgrade from Ubuntu 14.04 to 16.04, you can use the ubuntu tool update-manager. This tool make a clean upgrade from the system.
Please before update your system.
sudo apt-get update
sudo apt-get dist-upgrade
If you are done with the upgrade, install the update-manager-core.
sudo apt-get install update-manager-core
Now upgrade to the new ubuntu release.
sudo do-release-upgrade -d