At the time of writing this book, the actual Ubuntu server version is 14.04 Long Term Support (LTS). So, this recipe covers installation on 14.04.3; however, things are probably not going to change much in the next few releases, and this is applicable to older versions of Ubuntu as well.
Make sure that you have
root access to the server. If you don't have them, you may want to jump to Installing from a source on Ubuntu recipe, which explains how to install and run Redmine from the user's home directory.
sudo apt-get update && sudo apt-get upgrade