Ajax Form

Exemple : Field 'username'

//Form
    $form['username'] = array(
      '#type' => 'textfield',
      '#title' => 'Your user name',
      '#description' => 'User name',
      '#ajax' => array(
        'callback' => '::checkUserNameAjax',
        'wrapper' => 'edit-username',
        'method' => 'replace',
        'effect' => 'fade',
        'event' => 'change',
      ),

      //If you want to change the ID
      '#prefix' => '<div id="edit-username">',
      '#suffix' => '</div>',
    );

1. Ajax Methode to Edit Form
  function checkUserNameAjax($form, FormStateInterface $form_state) {
    $form['username']['#description'] = "YES";
    return $form['username'];
  }

2. Or using AjaxResponse to change HTML/CSS

  function checkUserNameAjax(array &$form, FormStateInterface &$form_state) {
    $valid = rand(0,1);
    if ($valid) {
      $css = ['border' => '1px solid green'];
      $message = ('User name ok.');
    }else {
      $css = ['border' => '1px solid red'];
      $message = ('User name not valid.');
    }
    $response = new \Drupal\Core\Ajax\AjaxResponse();
    $response->addCommand(new \Drupal\Core\Ajax\CssCommand('#edit-username', $css));
    $response->addCommand(new \Drupal\Core\Ajax\HtmlCommand('#edit-username--description', $message));
    return $response;
  }

Example 2 : Change a title using selected value

public function buildForm(array $form, FormStateInterface $form_state) {
  $form['methods'] = array(
    '#type' => 'select',
    '#title' => $this->t('Method'),
    '#default_value' => "",
    '#options' => ["a"=>"A Value","b"=>"B Value",],
    '#ajax' => array(
      'callback' => '::methodChangeAjax',
      'wrapper' => 'edit-fieldsset',
      'method' => 'replace',
      'effect' => 'fade',
      'event' => 'change',
    ),
  );
//Ajax changable fields container
  $form['fieldsset'] = array(
    '#type' => 'fieldset',
    '#title' => $this->t('Fields'),
    '#default_value' => "",
    '#prefix' => '<div id="edit-fieldsset">',
    '#suffix' => '</div>',
  );
  return $form;
}
function methodChangeAjax($form, FormStateInterface $form_state) {
  $method = $form_state->getValue('methods');
  $form['fieldsset']['#title'] = $method;
  return $form['fieldsset'];
}