Hi Kristen

You can add the amended header.php to the child theme, that way it would load that instead of the header.php in the updated theme. That would be the easiest way.

Or, yo could add a hook in the functions.php of your child theme to add content/changes into the header.php of the main theme. This is more advanced, see – http://wordpress.stackexchange.com/questions/43672/how-to-add-code-to-header-php-in-a-child-theme