how to update firebase document field angular

updateDoc(_id: string, _value: string) {

  let doc = this.afs.collection(‘options’, ref => ref.where(‘id’, ‘==’, _id));

  doc.snapshotChanges().pipe(

    map(actions => actions.map(a => {

      const data = a.payload.doc.data();

      const id = a.payload.doc.id;

      return { id, …data };

    }))).subscribe((_doc: any) => {

     let id = _doc[0].payload.doc.id; //first result of query [0]

     this.afs.doc(`options/${id}`).update({rating: _value});

    })

}