how to update existing document in firebase code example

Example 1: firestore set a document

let data = {
  name: 'Los Angeles',
  state: 'CA',
  country: 'USA'
};

// Add a new document in collection "cities" with ID 'LA'
let setDoc = db.collection('cities').doc('LA').set(data);

Example 2: firestore update map

let initialData = {
  name: 'Frank',
  age: 12,
  favorites: {
    food: 'Pizza',
    color: 'Blue',
    subject: 'recess'
  }
};

let updateNested = db.collection('users').doc('Frank').update({
  age: 13,
  'favorites.color': 'Red'
});