How to migrate description of the site and the top menu

To move description, you can use something like:

#site-title-wrapper {
	float: right;
}

I would add it under media query for 960px and up (style.css section: 12.2 Minimum width of 960 pixels)

In order to move menu, you will have to copy header.php to your child theme and move whole code block:

<nav id="site-navigation"...

...

</nav><!-- #site-navigation -->

below the line:

<!-- Custom Header - End -->