Why are we not able to update story points if it is in progress state.

                          I have created plan for product backlog,release and sprint backlog.I have used story work item and assigned some story points to that,but I am not getting how this story points get updated and when it is in-progress state then why it is not updated,according to my understanding story points get update only when story work item  is in complete state.Please let me know as soon as possible as I need it urgently.

Thanks in advance.