Here's a quick guide on upgrading to Ubuntu 19.04.
- Open Software & Updates.
- Open Updates tab and select For any version near the Notify me of a new Ubuntu version.
- Open Software Updater.
- Click on Install now.
- Click on Restart now.
Probably you will need to repeat 3, 4 and 5 steps few more times (is case you have an uninstalled upgrades or an <18.10 Ubuntu version):