Thank you for your quick reply!
It's version 2.3.4. I apologize for not including this before.
Yes, I have the admin access. Yes, I know how to transfer files. Thank you for asking these questions!
The customer wanted me to build a WooCommerce site for them, which I have done and it is live. That is why there are no products in the OSCommerce site. He has deleted them all.
So he wants me to change the opening page so it has a message and link to the new shopping cart.
I found out how to change the Contact page - from earlier forum posts - but cannot find out how to change the opening page. I'm sure it's because I don't know how to properly research what file controls this!