Display Post Except on Blog Template Pages

I'd like to change the loop on the Blog Template pages so that it displays an excerpt of the post, instead of the whole post. Could you let me know how to change this?

Original Code:
<?php if( $loop->have_posts() ): ?>
<?php while( $loop->have_posts() ): $loop->the_post(); ?>
<?php get_template_part('content',''); ?>
<?php endwhile; ?>


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