Fieldset preprocess

/**
*  Implements template_preprocess_fieldset()
*/
function EXAMPLE_preprocess_fieldset(&$variables) {
  if (isset($variables['element']['#id'])) {
    $id = $variables['element']['#id'];

    if ($id == 'edit-payment-information-payment-method') {
      $variables['legend']['title']['#markup'] = 'Custom markup';
    }
  }
}