How Update item on the second page in WebForm wizard
When you are in the preview page, you're too late then. 

You need to set this in your previous step in a validate function

function webform_certificadoemail_webform_element_alter(array &amp;$element, FormStateInterface $form_state, array $context) {

   if ($form_state-&gt;get('current_page') == 'step_1') { // or whatever you called the first page
     $element['#element_validate'][] = 'webform_certificadoemail_set_valor';
  }

}

function webform_certificadoemail_set_valor($element, FormStateInterface $form_state) {
   $form_state-&gt;setValue('valor','This value is .......');
}