Try hook_views_pre_render().

/**


Implements hook_views_pre_render().
*/
function cv_views_pre_render(&$view) {
// dpm($view); // dpm view here to see its properties.
// and replace $view->name and $view->current_display with actual values from your view.
// Note that my example uses Global: Text area for header, that's why I use $view->header['area'].
// Other options available for header have different names. Try to find one that you use.
if ($view->name == 'homepage' && $view->current_display = 'homepage_page_display') {
$view->header['area']->options['content'] = t('TEST message');
}
}

Use hook_views_pre_render().

/**
 * Implements hook_views_pre_render().
 */
function cv_views_pre_render(&$view) {
  // dpm($view); // dpm view here to see its properties.
  // and replace $view->name and $view->current_display with actual values from your view.
  // Note that my example uses Global: Text area for header, that's why I use $view->header['area']. 
  // Other options available for header have different names. Try to find one that you use.
  if ($view->name == 'homepage' && $view->current_display = 'homepage_page_display') {
    $view->header['area']->options['content'] = t('TEST message');
  }
}