title: mongoDB : renaming column name in collection
body: <p>I have a schema that looks like  </p>
  
  <pre><code>name: 
  value:
  pattern:
  XUknown:
  </code></pre>
  
  <p>I have 2 million documents in this collection.</p>
  
  <p><strong>Want</strong><br>
  - I want to rename the column name <code>XUknown</code> to <code>XString</code>, so that the schema looks like  </p>
  
  <pre><code>name: 
  value:
  pattern:
  XString:  
  </code></pre>
  
  <p>How can I achieve this?  </p>
  
  <p>Thank you</p>