We have to use a little trick! In other words, we will remove the meta box and then we will add a new one again:

function customposttype_image_box() 
{
  // remove
  remove_meta_box( 'postimagediv', 'post', 'side' );

  // add
  add_meta_box('postimagediv', 'Here we here', 'post_thumbnail_meta_box', 'post', 'side', 'low');
  }
add_action('do_meta_boxes', 'customposttype_image_box');