How do I can change the default description of elements?
you can find the descriptions at user.module under user directory (in modules directory)

you can find those descriptions under user_account_form function 

Ex, E-mail address field description below.

$form['account']['mail'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('E-mail address'),
    '#maxlength' =&gt; EMAIL_MAX_LENGTH,
    '#description' =&gt; t('A valid e-mail address. All e-mails from the system will be sent to this address. The e-mail address is not made public and will only be used if you wish to receive a new password or wish to receive certain news or notifications by e-mail.'),
    '#required' =&gt; TRUE,
    '#default_value' =&gt; (!$register ? $account-&gt;mail : ''),
  );


But it's better to  Use hook_form_alter to achieve it