This can be done with hook_form_alter:
function MY_MODULE_form_alter(&$form, &$form_state, $form_id) {
//drupal_set_message("Form ID is : " . $form_id);
switch($form_id) {
case 'MY_FORM_ID':
// dpm($form);
// $form['NAME_OF_YOUR_FIELD'] - this is the field you want to edit
// check that the field is set before looping over values
if(isset($form['NAME_OF_YOUR_FIELD'][LANGUAGE_NONE])) {
// loop over each values passing $item by reference
foreach($form['NAME_OF_YOUR_FIELD'][LANGUAGE_NONE] as &$item) {
if(is_array($item) && isset($item['street_block'])) {
$item['street_block']['thoroughfare']['#title'] = 'Address Line 1';
$item['street_block']['premise']['#title'] = 'Address Line 2';
}
}
}
break;
}
}
I found the path to the variable I wanted to change by outputting the $form array to the message area using devel's dpm function, and then searching though the array for the value "Address 1" using the search krumo module.