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 } ?>