How can I change an array element in the $config array from a Controller?

[eluser]warpspasm[/eluser]
Good Morning!

In application/config/my.php

I have a

$config['my_element'] = 'abcdef';

From a controller in /application/controllers/my_cont.php

function mine()
{
// I'd need to set $config['my_element'] again
// I tried this but it did not work:

global $config;

$config['my_element'] = 'ghijk';

// But it did not work, $config['me_element'] stayed the original value in another function!

}

How can I do this?
Thank you