// check original function befor override
remove_action( 'woocommerce_shop_loop_item_title', 'woocommerce_template_loop_product_title', 10);

add_action( 'woocommerce_shop_loop_item_title', 'add_custom_title', 10);
function add_custom_title() {
    echo '<h2 class="woocommerce-loop-product__title">'.str_replace('ß', 'ss', get_the_title()).'</h2>';
}