title: Update Firebase document based on custom fields
body: <p>I got the following Firebase document:</p>
  
  <pre><code>{
    "customField1": "customValue1",
    "customField2": "customValue2",
    "fieldToChange: "valueToChange"
  }
  </code></pre>
  
  <p>I wish to know how to do two operations:</p>
  
  <ol>
  <li><p>Find the document which has "customField1" set to "customValue1" and update "valueToChange" to something else.</p></li>
  <li><p>Find the document which has "customField1" set to "customValue1" AND "customField2" set to "customValue2" and update "valueToChange" to something else.</p></li>
  </ol>
  
  <p>How can I achieve this?</p>
  
  <p>I only see options where you need the docId:</p>
  
  <pre><code>const uploadCollection = this.db.collection&lt;any&gt;('uploads');
      return await uploadCollection.doc(docId).update({ 'fieldToChange': 'another value' });
  </code></pre>