Honestly, it would be redundant for me to put the guide here, so follow this guide: https://tutorials.ubuntu.com/tutorial/tutorial-install-ubuntu-desktop#0. It is built for installing natively on a machine with a USB drive, but if you ignore the beginning steps and start at step #5 then you'll be good.
There are a few ways to do this, but here are the steps which you need to complete
1.) Run sudo apt-get update && sudo apt-get upgrade -y
in your terminal in order to update any existing programs installed on the vm