function update_gallery_slug( $args, $post_type ) {
    if ( 'playbit_gallery' === $post_type ) {
        $my_args = array(
            'rewrite' => array( 'slug' => 'your-choosen-slug', 'with_front' => false )
        );
        return array_merge( $args, $my_args );
    }
    return $args;
}
add_filter( 'register_post_type_args', 'update_gallery_slug', 10, 2 );