title: How to change only the commit message of a commit in a branch with an existing pull request
body: <p>How can I change and push only the commit message of a commit already pushed on github.com in a branch with an existing pull request?I tried to use <code>git commit --amend</code> followed by <code>git push origin BRANCH</code>.</p>
  
  <pre><code>error: failed to push some refs to 'https://github.com/x/y'
  hint: Updates were rejected because the tip of your current branch is behind
  hint: its remote counterpart. Integrate the remote changes (e.g.
  hint: 'git pull ...') before pushing again.
  hint: See the 'Note about fast-forwards' in 'git push --help' for details.
  </code></pre>
  
  <p>How do I have to use <code>git pull</code> before using <code>git push</code>?</p>