How to add excerpt in loop in twentyeleven theme

Here is my loop code located in index.php file. I am using wordpress twentyeleven theme.

<?php if ( have_posts() ) : ?>

        <?php /* Start the Loop */ ?>
        <?php while ( have_posts() ) : the_post(); ?>
            <?php get_template_part( 'content', get_post_format() ); ?>
        <?php endwhile; ?>

        <?php twentytwelve_content_nav( 'nav-below' ); ?>

Where should i add the the_excerpt or any excerpt code into this?