Simply place a value to the query_string to select specific categories before the loop begins.
<?php query_posts($query_string . '&cat=5'); ?>
In the above example this would select category ID 5.
Below I have set the query to look at a category slug named press-relase.
<?php if ( have_posts() ) : ?>
<?php query_posts($query_string . '&category_name=press-release'); // Pull only date results from the slug 'press-release' ?>
<?php /* Start the Loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
<?php get_template_part( 'excerpt', get_post_format() ); ?>
<?php endwhile; ?>
<?php else : ?>
<?php get_template_part( 'content', 'none' ); ?>
<?php endif; // end have_posts() check ?>