How do I change the "Search…"-placeholder text

It’s probably because the code is already in your child theme’s functions.php file by default. Just search for the code in your functions file, it should look like this –

// Customize search form input box text
add_filter( 'genesis_search_text', 'custom_search_text' );
function custom_search_text($text) {
    return esc_attr( 'Search...' );
}

You can change the Search... part to whatever you need and then it should work fine.