You’ll need to create a new loop – outside of your main loop – using get_posts() and passing the page ID for the ‘About’ page and setting post_type to page Then, inside your new loop simply call get_the_excerpt()
Update:
From my links:
$args = array( 'post_type' => 'page' );
$myposts = get_posts( $args );
foreach ( $myposts as $post ) : setup_postdata( $post ); ?>
<li>
<?php echo get_the_excerpt(); ?>
</li>
<?php endforeach;
wp_reset_postdata();?>
</ul>
Should do the trick. I just made minor edits to the examples in the provided links. It’s explained better there than I can but it’s best to understand the ‘how’ and ‘why’ of this; there are important, basic WP concepts in use.