How do I make changes on dev server and copy to live?

Can I copy a existing live wp multisite (with 2 sites) to a development server, make changes to one of the 2 sites, and then copy back to the live site only the files and DB of the site I changed without affecting the site I did not change? (Each site has its own database but I don’t know how intertwined they are and if I might break something when I drop the existing DB on the live site I change and replace it with the DB tables from my new dev DB once its approved to go live). Has anyone done this?

I don’t want to copy the whole multisite back as I’m only updating only one site over many weeks and the other site may get content updates while I am developing. If I copy it all back – that second site will loose its updates.

My client has an existing WP multisite set up – 2 sites.

My task is to update one of those sites with a new theme and content while the old site remains live. I’m leaning towards creating a 3rd WP multisite and just building the site there and then making it the new site and redirect old to new.