To change the default Tagline or Sales Message in Zen Cart, you need to FTP to your server and download the header.php file. If you don't have an FTP client, try FileZilla. Its normally located under:
Make a backup copy of the file.
Open the file in a text editor. Find the section:
define('HEADER_SALES_TEXT', 'TagLine Here');
Then delete or change "TagLine Here". Be careful not to delete the single quotes at the start and end ( ' ). They are important.
Upload by FTP the modified file and overwrite the existing.