Normaly update() use to update existing firestore data. Review firestore docs for the same. In that given example they are updating population by increments of value or with new population number but before passing it in each update function values are stored in one cost value if it is not static value. as Asked by @dharmaraj please edit your questions by posting with full code you can also read given firestore documentation for your own studies.

    import firebase from "firebase/app";  
    const app = firebase.initializeApp({});
     const firestore = app.firestore(); 
    const batch = firestore.batch(); 
    const newUserId = firestore.doc(userId);
    batch.update(orderRef, {
     userId: newUserId,
    });

Log newUserId value and see what are you getting into it.

You can't store the reference object that doc() returns, it's an object that may have circular references and functions in it. doc() is not the id of the document. If you want to get the id (which is a string), then:

const newUserId = firestore.doc(userId).ref.id;
batch.update(orderRef, {
 userId: newUserId,
});

I don't know why batch validate input should be a pure object. I tried to push reference type id inside nested object then it work well, yeah I know it already is a trick, but it work.

change:

batch.update(docRef, {
  user: firestore.collection('users').doc(userId)
})

to:

batch.update(docRef, {
  user: {
    id: firestore.collection('users').doc(userId)
  }
})