Migrate from folder to domain root

Hello,

I’m really new to WordPress. I recently took on a client who wanted me to manage his small business site (brick and mortar) for him. I believed it was WordPress and I could use pagebuilder so I wasn’t too concerned. Well, it turns out it was static and created by some spawn of Satan software that made the code horrendous. I’d like to rebuild the website with WordPress. I can do a one-click install to a folder on his site, i.e., Domain/new while I get it finished, but once he gives me the approval, how do I make it live on /Domain/? Is it as simple as copying all of the files form /new over to /domain/?

Sorry for the stupid question, and again, sorry if this isn’t the right forum.

Thanks!