This blog post talks about how you can upgrade from Ubuntu 16.04 to Ubuntu 18.04.
Preparing to upgrade your Ubuntu distribution
Before performing any large upgrade to any computer system you should first back up any and all important work. This is because there is always a chance (no matter how small) that something may go wrong.
For Ubuntu you will likely want to back up your home directory and any services that store data not in this directory (such as databases).
This guide assumes you know how to back up your data and have done so before continuing. The upgrade from 16.04 to 18.04 is relatively painless but there is always a chance of something going wrong.
Upgrading to Ubuntu 18.04
The first thing to do is to make sure you have the latest updates and packages for your current distribution. This will ensure your system is fully up to date and may aid the upgrade process. This can be updated with the following two commands:
sudo apt update
sudo apt upgrade
Once these have finished your system will be fully up to date. It is recommended at this stage to reboot your computer to boot into the new kernel if one was installed.
By updating your Ubuntu installation to the latest packages there will be less for the upgrade process to do.
Once the system has been fully updated you can run the following command.
This will guide you through the process of upgrading your Ubuntu installation to 18.04. During the process it will download all the required files and upgrade the system. This may also require you to reboot your machine a couple times to fully complete the upgrade.
Following these steps will allow you to upgrade your Ubuntu machine and use the latest long-term support version.