Upgrade Ubuntu

Run your Ubuntu Update

Before you do anything, make sure that your system is already up-to-date. Run a full Ubuntu update and upgrade with Apt.

$ sudo apt update 
$ sudo apt upgrade
$ sudo apt dist-upgrade

This will help to ensure that the difference between packages is as small as possible. It also will be the way that Canonical has most likely tested the upgrade themselves, so it's least likely to encounter bugs. Lastly, before you begin to upgrade Ubuntu to 18.04 LTS you may also want to remove all no longer required packages using:

$ sudo apt dist-upgrade

How to Upgrade Ubuntu The Ubuntu Way

Ubuntu have developed their own automatic way of upgrading between releases. It essentially scripts the traditional Debian approach. This way, you can set it up to upgrade and walk away. Ubuntu will handle the rest. If you want to take this route, you're going to need to install one package first. So, do that.

$ sudo apt install update-manager-core

When that finishes, run the Ubuntu upgrade utility.

$ sudo do-release-upgrade

Ubuntu will ask you a couple of questions about how you want to handle the upgrade, and it'll start off upgrading your system. If you're doing this too soon(Bionic is still in beta), it will tell you that there is no release available. In that case, add the -d flag at the end of the command to pull a development release.

$ sudo do-release-upgrade -d