How to update a single firebase firestore document

After authenticating i'm trying to lookup a user document at /users/, then i'd like to update the document with data from auth object as well some custom user properties. But I'm getting an error that the update method doesn't exist. Is there a way to update a single document? All the firestore doc examples assume you have the actual doc id, and they don't have any examples querying with a where clause.

firebase.firestore().collection("users").where("uid", "==", payload.uid)
  .get()
  .then(function(querySnapshot) {
      querySnapshot.forEach(function(doc) {
          console.log(doc.id, " => ", doc.data());
	      doc.update({foo: "bar"})
      });
 })

You can precisely do as follows (https://firebase.google.com/docs/reference/js/v8/firebase.firestore.DocumentReference):

var db = firebase.firestore();

db.collection("users").doc(doc.id).update({foo: "bar"});

Check if the user is already there then simply .update, or .set if not:

    var docRef = firebase.firestore().collection("users").doc(firebase.auth().currentUser.uid);
    var o = {};
    docRef.get().then(function(thisDoc) {
        if (thisDoc.exists) {
            //user is already there, write only last login
            o.lastLoginDate = Date.now();
            docRef.update(o);
        }
        else {
            //new user
            o.displayName = firebase.auth().currentUser.displayName;
            o.accountCreatedDate = Date.now();
            o.lastLoginDate = Date.now();
            // Send it
            docRef.set(o);
        }
        toast("Welcome " + firebase.auth().currentUser.displayName);
    });
}).catch(function(error) {
    toast(error.message);
});

in your original code changing this line

doc.update({foo: "bar"})

to this

doc.ref.update({foo: "bar"})

should work

but a better way is to use batch write: https://firebase.google.com/docs/firestore/manage-data/transactions#batched-writes

-- UPDATE FOR FIREBASE V9 --

In the newer version of Firebase this is done like this:

import { doc, updateDoc } from "firebase/firestore";

const washingtonRef = doc(db, "cities", "DC");

// Set the "capital" field of the city 'DC'
await updateDoc(washingtonRef, {
  capital: true
});

You only need to found official ID of document, code here!

    enter code here
    //Get user mail (logined)
    val db = FirebaseFirestore.getInstance()
    val user = Firebase.auth.currentUser
    val mail = user?.email.toString()

       //do update
       val update = db.collection("spending").addSnapshotListener { snapshot, e ->
                val doc = snapshot?.documents
                doc?.forEach {
                    //Assign data that I got from document (I neet to declare dataclass) 
                    val spendData= it.toObject(SpendDt::class.java)
                    if (spendData?.mail == mail) {
                        //Get document ID
                        val userId = it.id
                        //Select collection
                        val sfDocRef = db.collection("spendDocument").document(userId)
                        //Do transaction
                        db.runTransaction { transaction ->
                            val despesaConsum = hashMapOf(
                                "medalHalfYear" to true,
                            )
                            //SetOption.merege() is for an existing document
                            transaction.set(sfDocRef, despesaConsum, SetOptions.merge())
                        }
                    }

                }
            }
}
data class SpendDt(
    var oilMoney: Map<String, Double> = mapOf(),
    var mail: String = "",
    var medalHalfYear: Boolean = false
)

correct way to do this is as follows; to do any data manipulation in snapshot object we have to reference the .ref attribute

 firebase.firestore().collection("users").where("uid", "==", payload.uid)
  .get()
  .then(function(querySnapshot) {
      querySnapshot.forEach(function(doc) {
          console.log(doc.id, " => ", doc.data());
          doc.ref.update({foo: "bar"})//not doc.update({foo: "bar"})
      });
 })