How can I add a title to a field in the checkout page?
Not possible in User Interface (via link or menu or whatever). But if you use the following code in a module, you can add a title to your form element:

&lt;?php
function CUSTOM_form_alter($form, $form_state, $form_id) {
  // You should probably check for a form_id in here, as this runs on every form.
  // Below you will need to find the part of the $form array that has the billing address.
  $form['[see note above]']['widget'][0]['address']['address_line2']['#title'] = t('Address 2');
}