Upgrade Ubuntu 14.04 to Ubuntu 16.04 – Server Upgrade

check HDD space
df -h
check Ubuntu version
cat /etc/*release
make your server system up-to-date
$ sudo apt-get update && sudo apt-get dist-upgrade
Then reboot your system to finish installing updates.
$ sudo init 6
install the update-manager-core package using the command below that is if it is not yet installed on your server:
$ sudo apt-get install update-manager-core
edit this file, /etc/update-manager/release-upgrades using your favorite editor and set Prompt=lts as in the output below:
$ sudo vi /etc/update-manager/release-upgrades
keep "next" and "yes" for this process
$ sudo do-release-upgrade   or    $ sudo do-release-upgrade -d
reboot the server and the server will be updated to Ubuntu 16.04
$ sudo init 6