Rewrite labels on fields rendered via php
The label is usually in the #title key in each content member of the array, so something like

$content['field_product_reference']['#title'] = t('Something new');
print render($content['field_product_reference']);


should update it.

If not, try installing the Devel module and running dpm($content['field_product_reference']); to see whereabouts in the array the label value is.