replace an existing website with WordPress

Hello.

Simple question: I have an old website which has written directly in HTML.
I’d like to replace it (same url) with a wordpress website.

But I’d like to put all the content of the old site in the new. While I am doing this operation I want the old site to stay online.

Question: how can I work separetely with the worpress installation leaving the old site running for a while and how – when the wordpress site is ready and complete – replace the old one with the new one making it available typing the same url?

Thanks in advance for your attention and your replies.
Andrea