Get parent page url to show up when it is in child pages

You can use something like this to get the parent page URL (and show its page title):

<?php if ( $post->post_parent ) { ?>
 <a href="https://wordpress.stackexchange.com/questions/192895/<?php echo get_permalink( $post->post_parent ); ?>" >
    <?php echo get_the_title( $post->post_parent ); ?>
 </a>
<?php } ?>

If you are running this code outside of the loop (thanks @BorisKuzmanov), then use this:

<?php global $post;
  if ( $post->post_parent ) { ?>
    <a href="https://wordpress.stackexchange.com/questions/192895/<?php echo get_permalink( $post->post_parent ); ?>" >
    <?php echo get_the_title( $post->post_parent ); ?>
    </a>
<?php } ?>