はじめに、Homebrew をアップデートします。

$ # macOS Sierra (10.12) + Homebrew
$ brew update

carthage の更新があるかどうかを確認します。 brew outdated で表示される場合、パッケージが更新できます。

$ brew outdated
carthage (0.23.0, 0.25.0) < 0.26.2

carthage の最新版をインストールします。

$ brew upgrade carthage
==> Upgrading 1 outdated package, with result:
carthage 0.26.2
==> Upgrading carthage
==> Downloading https://homebrew.bintray.com/bottles/carthage-0.26.2.sierra.bott
==> Pouring carthage-0.26.2.sierra.bottle.tar.gz
==> Caveats
Bash completion has been installed to:
  /usr/local/etc/bash_completion.d

zsh completions have been installed to:
  /usr/local/share/zsh/site-functions
==> Summary
🍺  /usr/local/Cellar/carthage/0.26.2: 75 files, 25.9MB

参考資料