i have duplicate site name issue in google search result , and to fix this issue i need to edit header.php

as this solution :

<title><?php wp_title('|', true, 'right'); ?></title>

Replace the title line with

<title><?php wp_title(''); ?></title>

but the problem that you know i cant edit the header.php because the theme security .

how i can edit that file ?

thank you