I have engaged an expert to build a website for me but I want to backup my old Wix, just in case.
There really is no way to externally backup a Wix site. Wix reliably keeps all versions of sites and you can always restore from the History option in the Dashboard.
If you want, you can export your database collections and keep those backed up externally.
If you need further details or clarification, I would recommend contacting Wix Customer Care , as they know best.