Creating second WordPress website in subdirectory

Hi there

I have been researching for a while but not quite figured out the best way to do this in my situation. I thought it shouldn’t be too difficult!

I have a clients wordpess website in which they want a total revamp including new content and theme. While I am setting up the new site they need to be able to access it and with no downtime on their current site.

I thought maybe the best way to do this would be to set up a subdomain with new database and work from there until finalised. But then I am a little confused on how to switch it over to be live. The current site is in the root domain. Will I need to delete the current site files once the new site is ready and just follow steps that are in “setting up site from new directory”?

Thanks for your help! 🙂