I have my zencart located on a subdomain and want to keep it there. How do I make the "home" button in the navigation bar point to my main domain?

For example my website is located at www.pizza.com. They can click a link to order now and it takes them to ordernow.pizza.com. I want to keep it this way as not everyone that comes to my site wants to order. But once I get them onto the zen-cart store I can't seem to navigate them back to my main website.

Thanks,