System: Mac OS Sierra (10.12.6)
As mentioned here, to update a go version you will first need to uninstall the original version.
To uninstall, delete the /usr/local/go
directory by:
$ sudo rm -rf /usr/local/go
Go to the downloads page and download the binary release suitable for your system.
To extract the archive file:
$ sudo tar -C /usr/local -xzf /Users/alexander/Downloads/go1.10.darwin-amd64.tar.gz
$ echo $PATH | grep "/usr/local/go/bin"