title: Update MongoDB collection using $toLower
body: <p>I have an existing MongoDB collection containing user names. The user names contain both lower case and upper case letters.</p>
  
  <p>I want to update all the user names so they only contain lower case letters. </p>
  
  <p>I have tried this script, but it didn't work</p>
  
  <pre><code>db.myCollection.find().forEach(
   function(e) {
   e.UserName = $toLower(e.UserName);
   db.myCollection.save(e);
   }
  )
  </code></pre>