Display Subdomain Install as Root

Hi. Not sure that Title is accurate, and I’m sure this has been addressed before but I couldn’t find this specific question. So let me explain simply.

I have a WP installation at a subdomain sub.example.com that I’ve been using for some time now.

The example.com content is being moved to old.example.com and I want the content at sub.example.com to display when someone visits example.com.

How do I get example.com to display my WP install at sub.example.com?

Thanks.