brew upgrade --cask --greedy. Everything is fine except for "google-back-and-sync". Brew says:
Error: google-backup-and-sync: It seems there is already an App at '/usr/local/Caskroom/google-backup-and-sync/latest/Backup and Sync.app'.
I don't understand what the problem is and how I can solve this.
To address this, I just did
rm -r /usr/local/Caskroom/google-photos-backup-and-sync/latest/Backup\ and\ Sync.app
brew reinstall google-photos-backup-and-sync