Modifying a query before it begins

functions.php

<?php
// Don't include the opening php tag

// Modify a query conditionally in functions.php
function prefix_modify_query( $query ) {
    if ( $query->is_home() && $query->is_main_query() && !is_admin() ) {
        $query->set( 'post_type', 'my_cpt_name' );
    }
}
add_action( 'pre_get_posts', 'prefix_modify_query' );