You have edited this file and I hope that you are using the overrides for your template over editing the core file. If so, you can just delete the override file and Zen Cart will use the default file. If not, please read about creating your own template.

From your expanded zip for Zen Cart, retreive the file tpl_header.php and replace the file with an unedited one.

Then in /includes/languages/english/header.php find this code:
Code:
define('HEADER_ALT_TEXT', 'Powered by Zen Cart :: The Art of E-Commerce');
and change the red highlighted text to:
Code:
define('HEADER_ALT_TEXT', 'THE HOMEPAGE LINK');
Or any other title that you want

Note: maintain the SINGLE quotes around the text