Hi everyone! I need some advice on migrating a website.

I've redesigned a non-profit's website locally using LocalWP, and now I need to replace the current live website with the new version.

The live website is part of a WordPress multisite network with four sites. I only want to replace the main site, keeping the same domain and leaving the other three sites untouched.

I have WordPress admin access, but no hosting control panel access. I've looked into UpdraftPlus, but I'm unsure whether it's suitable for this type of migration.

What's the safest and simplest way to migrate my new standalone WordPress site into just one site within the existing multisite network, without affecting the others?

I'm not an experienced developer, so I'd really appreciate any advice. Thanks!

EDIT
The client has confirmed that the other three sites in the multisite network are old, unused projects. They only need to be kept offline for archival purposes, not publicly accessible. Would it be simpler to archive the three old sites separately, outside the multisite network, rather than keeping them in it? I want to make sure they can be preserved safely and kept offline without affecting the migration of the main site.