title: How do I modify a specific commit?
body: <p>I have the following commit history:</p>
  <ol>
  <li><code>HEAD</code></li>
  <li><code>HEAD~</code></li>
  <li><code>HEAD~2</code></li>
  <li><code>HEAD~3</code></li>
  </ol>
  <p><code>git commit --amend</code> modifies the current <code>HEAD</code> commit. But how do I modify <code>HEAD~3</code>?</p>