You have a very old, completely table-based template:

@version $Id: stylesheet.css 4387 2006-09-04 13:54:28Z drbyte $

which looks like a Template Monster template. There will be little we can do to help you, because TM (if it is theirs) messes around with so many files and disables many admin functions that the standard answers don't apply.

The fine print on the home page says:

"This content is located in the file at: /languages/english/html_includes/YOUR_TEMPLATE/define_main_page.php

You can quickly edit this content via Admin->Tools->Define Pages Editor, and select define_main_page from the pulldown.

NOTE: Always backup the files in /languages/english/html_includes/your_template"

In your admin > Tools > Server/Version Info, what does it say about the Zen Cart version?