How can I update a single field in Firestore?

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To update a single field in Firestore, you can use the update() method on the document reference with the new value. Here is an example:

// Get a reference to the document you want to update
const docRef = db.collection('cities').doc('LA');

// Update the population field with a new value
docRef.update({ population: 4000000 })
  .then(() => {
    console.log('Document updated successfully');
  })
  .catch((error) => {
    console.error('Error updating document', error);
  });

This code updates the population field of the document with ID LA in the cities collection to the new value of 4000000. You can replace the field name and value with the ones you want to update.