I want to edit it on my laptop as a child theme and transfer the files to my server. I don’t know how

You’ve got a few hefty questions there so I’ll point you in the right direction.

  1. Run wordpress locally. You need MAMP (for mac) or WAMP (for windows). It’ll work just like your live site but the files will be on your computer. More info: https://codex.wordpress.org/Installing_WordPress_Locally_on_Your_Mac_With_MAMP

  2. Make a child theme for your site. This will allow you to make edits to your theme without changing the actual main files of your theme. It’s just safer. You never want to change the original. That way your changes will be safe through updates and plenty other upsides. You can just google “child themes wordpress” to get WordPress’s instructions on this.

  3. And finally, being able to do these first two steps in a way that lets you elegantly push your changes to your live site – Basically, you need to make a git repository. A git repo is a central place for you to store all of your files. It’ll track every change you make and it simplifies updating files across multiple locations (in your case your local machine and your server). It’s not as complicated as it sounds but it will take a bit to figure it out. Here’s more info: https://wordpress.org/support/topic/working-locally-and-pushing-to-remote-server

Just so you know, it’s great that you’re wanting to do this the right way. The way I briefly outlined above is definitely the way many professionals do it, so if you can get all this working you will be set!