Adding a custom.css stylesheet to your Genesis child theme (will work for non Genesis themes as well). First create a custom.css file with your custom styles and FTP it up to your child theme directory. Then add the below code to your child theme's functions.php file.

functions.php

<?php
// Do not copy opening php tag above

add_action( 'wp_enqueue_scripts', 'wsm_custom_stylesheet', 20 );
function wsm_custom_stylesheet() {
    wp_enqueue_style( 'custom-style', get_stylesheet_directory_uri() . '/custom.css' );
}

Three questions:
1 - This file will be deleted when I update my theme?
2 - Will that function deleted once I update the theme?
3 - If yes, is not more easy to just edit the Child Theme style.cc and upload it after each time I upload the theme?

Thanks!