switch domain names

Hey all!

Quick question and thank you in advance!  Last week (with help of tech support, which was awesome), I was able to delete one of my three sites I have on Homestead to build a completely new site; tech suggested I keep the existing domain in place and build the new site using it.

I bought a new domain to replace the old one, and the new site is ready--how do I switch them out?  

Probably a stupid question but I appreciate the help and hope it's easy!

JP