Yes you can edit the files directly if you want. That’s what most people do. The majority of the updates for Child themes are usually small and not necessarily something everyone would need to do if they didn’t want to. If I were you, I wouldn’t worry too much about it, I’d just edit your files however you feel comfortable.
If you don’t want to edit the files directly, then you could create your own custom css file and add in your edits there so that it stays separate from your child theme and won’t get overwritten. Just create a custom.css file, add it to your theme folder and make sure and link to that file from your style.css file.