If you use Homebrew to install software on macOS, you may notice that Homebrew may perform version upgrade prior to installing the software. If you wonder why, deep inside the core logic of brew, the main command line tool of Homebrew, there is a subroutine to perform automatic update when brew detects that the version installed on your machine is older than the latest available version. The automatic update is not usually a problem when your machine runs on latest macOS version. For older versions of macOS, however, automatic brew update can break things including brew itself. In the event that brew stops working after unexpected update, is it possible to rollback the update? In this article, we will show you how to undo brew update and bring back your system to the last working configuration.