How do I modify the email maximum length to 320 characters?
You can using hook_element_info_alter.
mymodule.module
/*
 * Implements hook_element_info_alter().
 */
function mymodule_element_info_alter(array &amp;$info) {
  if (!empty($info['email'])) {
    $info['email']['#maxlength'] = 320;
  }
}

Flush all caches so hook gets pickedup.
As Neograph734 pointed out, the mail column's VARCHAR length will also have to be increased in the DB.  See Resizing Fields in Drupal 8 Without Losing Data to learn how you can accomplish that.