title: Rename nested field in BsonDocument with MongoDB.Driver
body: <p>I have a document, it contains a CustomFields document, in which there are still documents.</p>
  <p>How to correctly update the <code>Checkpoint Comment</code> document name to the <code>Checkpoint Comment-test</code>.</p>
  <p>I tried as below but it doesn't work.</p>
  <pre class="lang-cs prettyprint-override"><code>var filter = Builders&lt;Visit&gt;.Filter;
  
  UpdateDefinition&lt;Visit&gt; update = Builders&lt;Visit&gt;.Update.Rename(x =&gt; x.CustomFields.FirstOrDefault(f =&gt; f.Name == &quot;Сheckpoint Comment&quot;).Name, &quot;Сheckpoint Comment-test&quot;);
  Collection.UpdateMany(filter.Where(x =&gt; true), update);
  </code></pre>
  <pre><code>{
    &quot;_id&quot;: ObjectId(&quot;61e5707d23516865481ebee7&quot;),
    &quot;CustomFields&quot;: {
      &quot;Сheckpoint Comment&quot;: {
        &quot;FieldId&quot;: ObjectId(&quot;6149d5de44175b8324482904&quot;),
        &quot;Type&quot;: &quot;Text&quot;,
        &quot;ValueBson&quot;: &quot;test&quot;
      },
      &quot;Time of arrival at the checkpoint&quot;: {
        &quot;FieldId&quot;: ObjectId(&quot;6149d5de44175b8324482905&quot;),
        &quot;Type&quot;: &quot;DateTime&quot;,
        &quot;ValueBson&quot;: null
      }
    }
  }
  </code></pre>