I'm using a Twenty Twelve theme with a child theme. When the screen is shrunk to less than a certain width a responsive menu is toggled. The problem is that this menu's top level pages look the same as nested ones so I want to modify the menu's template.
I thought this would be a matter of copying the menu's template to my child-theme but the template file is located in wp-includes (it's nav-menu-template.php I belive). How should I modify it so my changes are not erased when I update wordpress? And also with minimum risk of breaking something