[xubuntu] Upgrading from Xubunu 14.04 LTS to 16.04 LTS

GUI or command line.
GUI follow the software updater

Command line is:
sudo apt-get update
sudo apt-get dist-upgrade

If you have customised the distribution in a way you want to keep you would likely be better replacing dist-upgrade with just upgrade

You will unfortunately have to go through the tedious process of 4 upgrades though 14.04->14.10->15.04->15.10->16.04
You may want to instead just download an image and blow it all away.