2 domains on Squarespace account how do I switch which one I point it to?

Site URL: https://www.1400lavaca.com/

I am helping a client who has a website hosted by Squarespace make some changes to their website. While I understand how to edit pages / content, I am unfamiliar with where to change where a domain points to using Squarespace.  Both domains are with Squarespace but one of them points to another (A goes to B).  They want that switched (B going to A).  How do I do that?  
I am using to using other hosts interface with name servers etc. 

See this guide: Moving a domain to another Squarespace site.

If you need further help, you can chat online to Squarespace Customer Care, who can walk you through the process. You can reach out to them here .