Replace parent function

// replace parent function
function my_addition () {
    // your code goes here
}

function my_theme_setup () {
    remove_action( 'wp_enqueue_scripts', 'p2_iphone_style', 1000 );
    add_action( 'wp_enqueue_scripts', 'my_addition', 1000 );
}
add_action( 'after_setup_theme', 'my_theme_setup' );