How I can update CI_SESSION

Hello Programmer,

I need to update the CI_SESSION pre and post login but i couldn't find the solution how i can do that. I have tried like this

@session_start();
$this->session->set_userdata("CI_SESSION",session_id());

but it's not working. 

Please help me to sort this issue.

Thanks in advance. 

Girish Phalak