/**
 * Implements hook_form_FORM_ID_alter().
 */
function modulename_form_user_register_form_alter(&$form, &$form_state) {
  $form['account']['pass']['#type'] = 'password';
  $form['account']['pass']['#title'] = t('Password');
  $form['account']['pass']['#description'] = '';
}

Drupal 7