What is the process for modifying Firestore data in Flutter with customized identifiers such as email or ID?

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The process for modifying Firestore data in Flutter with customized identifiers such as email or ID would involve the following steps:

1. Set up a Firestore instance in your Flutter app by importing the necessary libraries and initializing a Firestore instance.

2. Access the collection that contains the data you wish to modify using its custom identifier such as email or ID.

3. Use the document() method to access a specific document within the collection, passing in the email or ID as a parameter.

4. Use the updateData() method to modify the data within the document. You can pass in a Map<String, dynamic> object to represent the data to be updated.

Here is an example code snippet that demonstrates the process:

import 'package:cloud_firestore/cloud_firestore.dart';

Firestore firestore = Firestore.instance;

void updateUserData(String email) {
  CollectionReference users = firestore.collection('users');
  DocumentReference userRef = users.document(email);

  userRef.updateData({
    'name': 'John Doe',
    'age': 30,
    'city': 'New York'
  });
}

In this example, we first access the users collection and then access a specific document using the email address as the document identifier. We then update the fields within the document using the updateData() method.