Change the default "Create new account" title on the User registration page
You can add the following two hooks to your theme's *.theme file or your module's *.module file. The first to alter the title that's displayed in the page title block. The second to alter the meta title.

/**
 * Implements hook_preprocess_HOOK().
 */
function MYTHEME/MYMODULE_preprocess_page_title(&amp;$variables) {

  if (\Drupal::routeMatch()-&gt;getRouteName() == 'user.register') {

    $variables['title'] = t('New Title');
  }
}

/**
 * Implements hook_preprocess_HOOK().
 */
function MYTHEME/MYMODULE_preprocess_html(&amp;$variables) {

  if (\Drupal::routeMatch()-&gt;getRouteName() == 'user.register') {

    $variables['head_title']['title'] = t('New Title');
  }
}