If you like to remove the search box then go to Appearance->Theme Options->Design Options->Custom Heade r and check Hide Searchform From Header
But if you want to keep something else in place of it then you have to edit the code. You can go to Clean Retina->library->Structure->header-extensions.php and make necessary changes but create child theme for making changes.
I’ve tried adding text to header-extensions.php but it won’t show on the site. Can you please tell me where in that file I would put text so that it shows just above the social media logos at the top-right?