Code snippet that can be used to change a field element in form using hook_form_alter in Drupal 8.
/**
* @param $form
* @param $form_state
* @param $form_id
*/
function mytheme_form_alter(&$form, &$form_state, &$form_id)
{
if ($form_id == 'FORM_ID'){
// to remove description
unset($form['name']['#description']);
// to update title
$form['name']['#title'] = t("Your Custom name");
// to update placeholder
$form['name']['#attributes']['placeholder'] = 'Custom placeholder';
// to add class
$form['name']['#attributes']['class'][] = 'custom_class';
// to add class to submit button
$form['actions']['submit']['#attributes']['class'][] = 'submit_class';
// to change value of submit button
$form['actions']['submit']['#attributes']['value'][] = t('Submit Value');
// to add prefix
$form['actions']['submit']['#prefix'] = '<div class="class_div_prefix">';
// to add suffix
$form['actions']['submit']['#suffix'] = '</div>';
// to update All option with your text
$form['field_name']['#options']['All'] = t('Select option');
}
}
