Open the index.js file in the functions directory.

Add this code to create a function that triggers on Firestore document changes:
```javascript
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

exports.onDocumentUpdate = functions.firestore
.document('your-collection/{docId}')
.onWrite((change, context) => {
// Get the before and after snapshots
const beforeSnapshot = change.before.exists ? change.before.data() : null;
const afterSnapshot = change.after.exists ? change.after.data() : null;

console.log("Document updated:", context.params.docId);
console.log("Before:", beforeSnapshot);
console.log("After:", afterSnapshot);

// Perform your custom logic here

return null;
});
```