"Super-Newb" question about uploading new WP theme


I’m “Super-Newb” bc I’m probably the least qualified person to be trying Flywheel and local. I have a website (WP theme) that I manage (the only one I’ve ever managed but I’ve been doing so since 2013) that I moved from Bluehost to Flywheel a few months ago, and I have a new WP theme I’d like to prepare in the background before switching out my old theme. I’m assuming (hopefully correctly) that this is what Local is perfect for.

Assuming so, I downloaded Local to my MB Air, but I can’t see where or how I would upload the new theme. Do I do it on the dashboard that my current theme lives on or do I upload it from my computer’s finder?

I’m sure more questions will follow. If anyone knows of a YouTube that answers this question I’d be appreciative.