Edit child-theme funtions.php

https://www.lofotenmusicfestival.no/wordpress
Using latest version of WP.
——

I have made a child theme for Twenty Twelve which is working fine.

I have made an empty functions.php in my child-theme-folder.
My question is how to edit the functions.php.

Just an example, if I want to change the function below, which is in the parent functions.php, and for instance remove the after_widget and before_widget, or change the h3 to h2 or whatever. I just want to be able to paste in the exact same code into the child-themes functions.php and make some changes.

How?

function twentytwelve_widgets_init() {
	register_sidebar( array(
		'name' => __( 'Main Sidebar', 'twentytwelve' ),
		'id' => 'sidebar-1',
		'description' => __( 'Appears on posts and pages except the optional Front Page template, which has its own widgets', 'twentytwelve' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget' => '</aside>',
		'before_title' => '<h3 class="widget-title">',
		'after_title' => '</h3>',
	) );