Before updating I recommend to make a backup copy of the important information.
Update the system to the current state:
sudo apt-get update sudo apt-get upgrade
Let’s see the current version:
We are ready to upgrade to version 16.04:
sudo update-manager -d
After the update is complete, you will need to reboot.
I will note that many packages will also be updated, for example PHP5 will be updated to PHP7, etc., so you need to be ready for this.
How to downgrade the PHP version I described in this article – How to install PHP 5.6 in Ubuntu 16
Or how to upgrade here – Upgrading PHP Version on Ubuntu 14.04