How to update some application to latest version with homebrew-cask? I noticed some packages provided by homebrew-cask uses a url of the latest version and a version number of "latest" such as dropbox and chrome. Thus they will never be upgraded with brew upgrade. Is there any simple way to update such packages using homebrew-cask? 

And if I call brew cask install dropbox again, it shows there already exist some cache of old version, which I feel so dumb.
Try this command:

brew cask install dropbox --force


Please consider logging a bug over at homebrew-cask about your concerns with updating this kind of cask.