If you are on Ubuntu 16.04 and need to move to 18, the following steps would help you achieve that. The upgrade is large and you would be inquired with a few questions while the up-gradation is happening.
First update the current distribution, with the following command
$ sudo apt update
$ sudo apt upgrade
This makes sure your Ubuntu is up to date. Next, upgrade the distribution of ubuntu
$ sudo apt dist-upgrade
This handles changing software dependencies with new versions of packages.
I then follow this up with:
$ sudo apt-get autoremove
This removes dependencies from uninstalled applications. You can do the same thing from the GUI desktop with the utility application Bleachbit. After that, it’s time to get things ready for the big upgrade with:
$ sudo apt install update-manager-core
$ sudo do-release-upgrade
This will start upgrading your system to 18.04. Along the way, Ubuntu will ask you several questions about how to handle the upgrade.