Ubuntu 18.04 LTS Bionic Beaver has just been officially released few days ago.
For those who wish to upgrade from your older version of Ubuntu to this new version but are still new to this distro, below is the tutorial on how you can upgrade your current Ubuntu installation.

The steps are very simple, just a few commands away. If you are expecting GUI way, sorry, I am more to a CLI person, so my tutorial will be only on CLI.

1. Make sure everything is updated.
Make sure everything in your current Ubuntu installation are updated.

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

2. Remove additional repositories.
You might be added some additional 3rd party repositories along the way you’ve been using your Ubuntu desktop/server.
Move all the .list files in /etc/apt/sources.list.d/ to somewhere you want to keep those files.
Check in your /etc/apt/sources.list if there are anything added manually or lines that are not pointed to the official Ubuntu mirror, and remove them.

3. Start to upgrade.
Install the package below.

$ sudo apt install update-manager-core

Finally, run the command below to start upgrading your Ubuntu installation.

$ sudo do-release-upgrade

There might be some questions popup during the upgrade, just make sure you are attended to the desktop/server during the whole process of upgrading.

4. For paranoid people.
For those who are paranoid and wanted to make sure the upgrade is perfect, run the commands below again to double check everything is properly upgraded and installed.

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

5. Reboot
Now, just reboot and you will be getting the Ubuntu 18.04 LTS running.