First, check if the problem was with your download. Delete the downloaded file (its location will be pointed out in the error message) and try again.

If the problem persists, the cask must be outdated. It’ll likely need a new version, but it’s possible the version has remained the same (happens occasionally when the vendor updates the app in place).

1. Go to the vendor’s website (brew home {{cask_name}}).
2. Find out what the latest version is. It may be expressed in the URL used to download it.
3. Take a look at the cask’s version (brew cask _stanza version {{cask_name}}) and verify it is indeed outdated. If it is:

Help us by submitting a fix. If you get stumped, open an issue explaining your steps so far and where you’re having trouble.