function example_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'user_login_block' || $form_id == 'user_login') {
    $form['actions']['submit']['#value'] = t('Sign in');
  }
}

function example_form_user_login_block_alter(&$form, &$form_state, $form_id) {
  $form['actions']['submit']['#value'] = t('Sign in');
}