public function buildForm(array $form, FormStateInterface $form_state) {
     ...
    if($form_state->getValue('g_actualPartIndex') == NULL){
        $form_state->setValue('g_actualPartIndex', 0);
    }
    $form = $this->setHeader($form, $form_state);

     $form['Next'] = array(
        '#type' => 'submit',
        '#value' => "Go to next step",
        '#ajax' => [
            'wrapper' => 'projekt-form-wrapper',
            'callback' => [$this, 'form_ajax_submit'],
            'method' => 'replace',
            'effect' => 'fade'
        ]
    );

    $form['#prefix'] = '<div id="projekt-form-wrapper">';
    $form['#suffix'] = '</div>';
    return $form;
}

public function form_ajax_submit($form, $form_state) {
    $form_state->setValue('g_actualPartIndex',
       (int)($form_state->getValue('g_actualPartIndex')) + 1);

    $form = $this->setHeader($form, $form_state);
    $response = new AjaxResponse();
    $response->addCommand(new HtmlCommand(
        '#projekt-form-wrapper',
         $form
    ));
    return $response;
}

private function setHeader(array $form, FormStateInterface $form_state){
    $actualPartIndex = $form_state->getValue('g_actualPartIndex');
    $form['header'] = array(
        '#type' => 'markup',
        '#markup' => t('<div id="generatorHeader">
            <span class="part '.($actualPartIndex == 0 ? 'active' : '').'">Part 1</span>
            <span class="part '.($actualPartIndex == 1 ? 'active' : '').'">Part 2</span>
            <span class="part '.($actualPartIndex == 2 ? 'active' : '').'">Part 3</span>
            <span class="part '.($actualPartIndex == 3 ? 'active' : '').'">Part 4</span>
            <span class="part '.($actualPartIndex == 4 ? 'active' : '').'">Part 5</span>
            <span class="part '.($actualPartIndex == 5 ? 'active' : '').'">Part 6</span>
        </div>'),
    );

    return $form;
}