It would help if you listed your $args array, but, the portion you are looking for is under ‘rewrite’. Here’s the way I like to build an $args array for registering a post type:
// set up the labels
$labels = array(
'name' => _x('Archived Jobs', 'post type general name'),
'singular_name' => _x('Archived Job', 'post type singular name'),
'add_new' => __('Add New'),
'add_new_item' => __('Add New Job'),
'edit_item' => __('Edit Job'),
'new_item' => __('New Job'),
'view_item' => __('View Job'),
'search_items' => __('Search Jobs'),
'not_found' => __('No Jobs found'),
'not_found_in_trash' => __('No Jobs found in Trash'),
'parent_item_colon' => '',
'menu_name' => 'Archives'
);
//set up the rewrite rules
$rewrite = array(
'slug' => 'archives'
);
//build the arguement array
$args = array(
'labels' => $labels,
'public' => true,
'publicly_queryable' => true,
'show_ui' => true,
'show_in_menu' => true,
'query_var' => 'archives',
'rewrite' => $rewrite,
'capability_type' => 'post',
'has_archive' => true,
'hierarchical' => false,
'menu_position' => null,
'supports' => array('title','editor','author','comments')
);
register_post_type('archive',$args);
The portion of this you are looking for is the $rewrite part, change the slug to whatever you want (url friendly of course, no weird characters or spaces). You may even wish to put slashes in there. In my example, a URL reads: www.website.com/archives/post_name/