^^^ too bad answers like that one can’t be downvoted. Problem solved.
I haven’t looked in the admin section to verify this, but I imagine you would go to the Domains page and do any reassignments there.
Thanks for asking @chrisrindone! we have lots of people that can learn from this so I would never downvote a question like this 
@gregraven that’s a great answer and it’s something someone should definitely do if they are switching which site their domain points to, but it’s actually not the entire truth.
A little “inside Netlify” here: We determine which site to send a request to based on which site has that domain on it, which can be different from the site that your DNS points to.
For example, if your site is awesome-site.netlify.com and you create a domain and setup the DNS on that domain with a CNAME record for awesome-site.netlify.com, it still won’t load that site unless you add that domain as a custom domain or domain alias on your awesome-site.netlify.com domain page. If you create a secondary site called super-site.netlify.com then you can leave the DNS the way it was and just remove the custom domain from the previous site and add it as a custom domain on super-site.netlify.com and it will load super-site.netlify.com even though the cname record is still for awesome-site.netlify.com. 
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thanks for writing such a super helpful answer @futuregerald