How do I change Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution?

look in includes -> languages -> english -> index.php and find this:

// This section deals with the "home" page at the top level with no options/products selected
/*Replace this text with the headline you would like for your shop. For example: 'Welcome to My SHOP!'*/
define('HEADING_TITLE', 'Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution.');
} elseif ($category_depth == 'nested') {
// This section deals with displaying a subcategory
/*Replace this line with the headline you would like for your shop. For example: 'Welcome to My SHOP!'*/
define('HEADING_TITLE', 'Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution.');
}

change the portions in red to whatever you want.
save the file to the correct folder and upload to the correct folder on your server.