You should use a custom page for this. For reference, we can modify the default (twentynineteen) post content block. Depending on your theme, you may need to modify single.php, or a content block included from there. In the twentynineteen theme, we can see this on line 24:
get_template_part( 'template-parts/content/content', 'single' );
Following that, we can look into template-parts/content/content-single.php at line 23:
23 the_content(
24 sprintf(
25 wp_kses(
26 /* translators: %s: Name of current post. Only visible to screen readers */
27 __( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentynineteen' ),
28 array(
29 'span' => array(
30 'class' => array(),
31 ),
32 )
33 ),
34 get_the_title()
35 )
36 );
37
So, you could replace this call to the_content with something like this:
the_excerpt(); // Echo the excerpt directly.
$excerpt = get_the_excerpt(); // Store the excerpt in $excerpt.
// Retrieve the "raw" exceprt - this has not passed any filters etc,
// and instead comes directly from the database.
$post = get_post();
echo $post->post_excerpt;