Changing a config variable after its set in config?

[eluser]delay[/eluser]
I am using ci sessions and I am trying to setup a remember me feature in my login. So in the config file I have

Code:

$config['sess_expiration'] = 7200

; The default setting of 7200 is good if a user doesn't select remember me. If they select remember me I would like to change it in the program to 0

Code:

$config['sess_expiration']=0

. How can I change a config setting from within a model?

Code:

if ($rememberme) {
        $config['sess_expiration'] = 0; //doesn't work
        }

Thanks for any help.