title: Firestore firebase Android search and update query
body: <p>I am trying to update a field of existing document in my collection in <strong>FireBase Firestore</strong>. I want to search that document using one of its field. 
  My collection name is "complaints", 
  My document's structure looks like --</p>
  
  <pre><code>complainant_Name: "XYZ"
  complaint_Details : "some random details"
  e_Mail: "xyz@gmail.com"
  id: 5
  phone_No: "1234567890" 
  </code></pre>
  
  <p>I want to search documents and if "id" field of a document is say 5, I want to update it say change "complainant_Name" field to "ABC". How to write the code for this query in java android? I couldn't find any code source which tells how to update after searching in <strong>Firebase Firestore</strong>.</p>