brew cask upgrade does not seem to work ~$ brew cask --version
Homebrew-Cask 1.6.6
Homebrew/homebrew-cask (git revision 1c78e; last commit 2018-06-01)
~$ 
~$ brew update
Already up-to-date.
~$ 
~$ brew cask outdated
~$ 
~$ brew cask info google-chrome
google-chrome: 67.0.3396.62
https://www.google.com/chrome/
/usr/local/Caskroom/google-chrome/66.0.3359.181 (2 files, 77.5KB)
From: https://github.com/Homebrew/homebrew-cask/blob/master/Casks/google-chrome.rb
==&gt; Name
Google Chrome
==&gt; Artifacts
Google Chrome.app (App)


Above is the output when I was trying to upgrade google-chrome. It looks like I have Chrome version 66, but the server already has version 67. However, I was not able to upgrade it with brew cask upgrade. I'm wondering whether I have done anything wrong.
There is no upgrade command available for brew cask. The GUI apps are updates by running Check for updates... from within the application which is generally found in the menu.

You can force install a newer version of the cask item by running brew cask reinstall &lt;package&gt;. In case of Google Chrome, run brew cask reinstall google-chrome.

You can use brew cask upgrade --greedy to upgrade all the GUI apps to respective latest version. From man brew-cask:


  upgrade [--force] [--greedy] token [ token ... ]
  
  Without token arguments, upgrade all the  installed  Casks  that
    have  newer  versions available in the tap; otherwise update the
    tokens given in the command line. If --greedy is given then also
    upgrade the Casks having auto_updates true or version :latest.