Swap domains

Hello forums,

I would need your advice for the following situation.

Situation:
– I have an existing website, let’s call it http://www.example.com. It was created with wordpress (org).
– I need to replace it with a fresh / brand new website.
– I have already built the new website using a new wordpress installation under test.example.com

What I want to do:
– Keep the old website intact (because it contains many valuable contents) and move it to archive.example.com where it will remain active
– Move the new website (built on test.example.com) to the main URL http://www.example.com

My question to you : how do I do those last two steps ? I can’t find relevant help on the interwebs, as most tutorials cover the case where the old website gets discarded.

Thanks in advance for your help.
Cedric