You can use your web browser to go to Drupal.org and download the newest version, or you can use any number of tools to do this for you. For example, you can use Drush, if you have it installed. (It is already installed and configured in the Vagrant profile).
$ drush dl drupal
The command will download and unpack the latest stable version of Drupal into its own directory. In the examples, though, I will assume that Drupal has been downloaded through a web browser or a command-line tool like wget, and has not yet been unpacked.