How can modify request.data in django REST framework

I am using Django REST Framework

request.data = '{"id": "10", "user": "tom"}'

I want to add extra attribute like "age": "30" before sending it to further like

    request.data = new_data    response = super().post(request, *args, **kwargs)

I have two issues

1. Why request.data is coming as string rather than dict
2. How can I update the request.data