title: Why doesn't yarn updated when running upgrade?
body: <p>I'm trying to use latest yarn version after upgrading it.<br>
  I've follow over <a href="https://yarnpkg.com/en/docs/install#mac-stable" rel="noreferrer">https://yarnpkg.com/en/docs/install#mac-stable</a> instructions, but yarn didn't really upgraded.<br>
  After running <code>brew upgrade yarn</code>:</p>
  
  <pre><code>$ brew upgrade yarn
  Updating Homebrew...
  Error: yarn 1.9.4 already installed
  </code></pre>
  
  <p>And after running <code>yarn --version</code>:</p>
  
  <pre><code>$ yarn --version
  1.9.2
  </code></pre>
  
  <p>Is there any way to use the latest version?<br>
  Although the guide says that <code>brew upgrade yarn</code> should do it automatically..</p>