Update Genesis Post Info with Last Updated date in place of Publish Date. This code can be added to the end of functions.php or you can create this file genesis-last-updated-post-info.php in wp-content/mu-plugins/

genesis-last-updated-post-info.php

// filter post_info to use 'Last updated' date
add_filter( 'genesis_post_info', 'fe_genesis_post_info_add_last_mod' );

/**
 * Change Genesis Post Info date to last modified date
 *
 * @since 0.1.0
 *
 * @param  string $post_info, string (with shortcodes) for post info
 * @return string Genesis default post info with modified instead of publish date
 */
function fe_genesis_post_info_add_last_mod( $post_info ) {

	$post_info =
		__( 'Last updated on', 'genesis-last-modified-post-info' ) . ' [post_modified_date]'
		. ' by [post_author_posts_link] [post_comments] [post_edit]';
	return $post_info;
}