Change twentytwelve sidebar responsive relocation?

The fast way to do this is to just change how it is loaded, open index.php and move this code:

<?php get_sidebar(); ?> 

From line 73 and put it under get_header(); ?> around line 18, so it should look like:

get_header(); ?>

<?php get_sidebar(); ?>

<div id="primary" class="site-content">
// rest of twentytwelve code.

Ps. I highly recommend you create a child theme so when you update the theme, the changes you make do not break.