CodeIgnter Session Update, How can I update CodeIgiter Session ????

[eluser]Unknown[/eluser]
Hello Guys,
I am in sort of trouble in CodeIgniter. I mean I can set the CI session variable at first time but cannot update it ?
My Code are :

Code:

$user = $this->users_model->verify_user($this->input->post('username'),$this->input->post('password'));

in which

Code:

$user

has following properties of

Code:

$id,$first_name,$username,$email

of object here and is assigned to the session variable

Code:

$this->session->set_userdata('user', $user);

now I cannot update it. If I have to access it I can access it by

Code:

$this->session->userdata('user')->id

and so on but cannot update it.
Please Help.!
Thanks in advance.