/**
 * Implements hook_user_view_alter().
 */
function {{ machine_name }}_user_view_alter(&$build) {
  // Check for the existence of a field added by another module.
  if (isset($build['an_additional_field'])) {
    // Change its weight.
    $build['an_additional_field']['#weight'] = -10;
  }

  // Add a #post_render callback to act on the rendered HTML of the user.
  $build['#post_render'][] = 'my_module_user_post_render';
}