How to show summaries, but not full text? (index.php is almost empty)

Hey there,

my blog is here: http://www.kai-waehner.de/blog
It shows every article with full text, but I want to show just the summary! otherwise there is no overview!

I found this the following options at “settings -> reading”:

For each article in a feed, show
– Full text
– Summary

I chose summary, but this does not change anything.

I also looked at the index.php, because I searched this forum: I shall replace the_content() with the_excerpt() here, but it does not contain much content to change:

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./wp-blog-header.php');
?>

Where can I configure the blog correctly?

Thank you for help.
Best regards, Kai.