Make a copy of includes\languages\english.php in includes\languages\YOUR_TEMPLATE, then find and edit the appropriate lines from the code below
Code:
// banner_box
  define('BOX_HEADING_BANNER_BOX','Sponsors');
  define('TEXT_BANNER_BOX','Please Visit Our Sponsors ...');

// banner_box - all
  define('BOX_HEADING_BANNER_BOX_ALL','Sponsors');
  define('TEXT_BANNER_BOX_ALL','Please Visit Our Sponsors ...');