how to update fields in firebase code example

Example 1: firebase cloud method update

firebase deploy --only functions:functionNameHere

Example 2: document.set() firebasefirestore java

var washingtonRef = db.collection("cities").doc("DC");// Atomically add a new region to the "regions" array field.washingtonRef.update({    regions: firebase.firestore.FieldValue.arrayUnion("greater_virginia")});// Atomically remove a region from the "regions" array field.washingtonRef.update({    regions: firebase.firestore.FieldValue.arrayRemove("east_coast")});