If you are using a child theme and need to edit any of the files, including the styles, you can do it directly in the Theme Editor that is found under the Appearance dashboard menu group.
Appearance >> Theme Editor
Normally when you open up a file, it’s going to be without line numbers and without colourized code like you see in the screenshot below:
To make it easier and better managed, I use the plugin WP Editor for this website. It enhances your editing experience much better than the default WordPress version.
From this screen, you can now view and edit your child theme style.css file. You can also navigate to other files in your child theme (see the right side column) to open and edit as well.
You can read more detail from the WordPress site for Editing Files.
IMPORTANT: Before you start editing any file, regardless if it’s the child theme, parent theme, or other, ALWAYS create a backup first whether it’s simply copying the files or using a Backup plugin (recommended).