You can access your CSS file via FTP but if you do not have access to FTP programs the instructions below will explain how to obtain the CSS file without using FTP.

Download CSS File
1) click store name

2) click "Head Tag content/css" tab

3) right click the path to the css file

4) click save as (Recommended: save 2 copies)

5) make your changes using a text editing program like notepad or textedit, or html & css editor

Upload CSS File

1) click store name

2) click "Head Tag content/css" tab

3) click "edit here" next to your css file

4) upload and browse to your file, click update

5) click update on the bottom right

Note: sometimes browsers will cache data in effort to load the page faster and can cause it to look like your changes have not been made. Clearing your cache usually resolves the issue.