BACKUP FILE BEFORE ALTERING
/includes/languages/english.php
Code:
// best_sellers box text in sideboxes/best_sellers.php
  define('BOX_HEADING_BESTSELLERS', 'Bestsellers');
  define('BOX_HEADING_BESTSELLERS_IN', 'Bestsellers in<br />&nbsp;&nbsp;');
Safest way is to create folder
i.e.
/includes/languages/nameofyourtemplate/
Place english.php into that folder.