How to Change the property of an Observable Object. Angular 2/BehaviorSubject

Question

I have an object of type Person which has fields of firstName, lastName, age. I detect for any change using Behavior Subject. I have an observing component that is subscribed to every change on this Person Object. Once the change detects the observing component will call a method. I want this method to change the property of the Observable Person Object.

In My Service

export class PersonService {

    personToCopySource = new BehaviorSubject<Person>(null);
    personToCopy = this.personToCopySource.asObservable();

In My Component

 export class ObservingComponent {

    constructor(public person: Person) {}

    ngOnInit(){
      this.personService.person.subscribe(
         data=> {
           this.updateMethod()
          }
      )
    }

   updateMethod(){
     this.personService.firstName = 'updated First Name';
   }

When I change the property of the Observing object I'm getting this error. "Property" firstName does not exist on type Observable