Change portfolio_item slug

HI
I have change the porfolio slug in file functions.php in line 555:

‘public’ => true,
‘has_archive’ => true,
‘rewrite’ => array(‘slug’ => ‘portfolio_item‘),
‘supports’ => array(‘title’, ‘editor’, ‘thumbnail’),
‘can_export’ => true,
‘show_in_nav_menus’ => true,

and also add the flush to rewrite rules

this doesnt work..
Pleas I need an orderer tutorial or instruction for that. Just change functions.php file?? or wich others??

thanks