Change Post Date to Last Updated

Good Afternoon,

I would like to display the date a post was last updated instead of the publish date. This is a code snippet I found that accomplishes this:

<p>Last Updated: <?php the_modified_date(); ?></p>

I tried modifying loop-single.php by replacing this:

<?php echo $td_mod_single->get_date(false);?>

with this:

<p>Last Updated:</p><?php echo $td_mod_single->the_modified_date(false);?>

This did not work and prevented the post content from displaying.

Can you please tell me the best way to change the post date to most recently modified date?

Thanks!