Unable to modify theme with style.css

This is my first wordpress. Version 3.5.1. I am using Twenty Twelve theme. My experience with cascading style sheets is limited.

path1 — Dashboard — Appearance — Editor — style.css — Update

When I use path1 to modify my style.css, I never see any changes to my website’s appearance.

path2 — Dashboard — Appearance — Background

When I use path2 to modify my website, I see changes.

I found an article that told me to use a child theme so I installed a child theme and realized (after I successfully installed and activated the child theme) that the css examples don’t work.

I selected and cut all the code and updated a blank style.css. The website looks unchanged.

What am I doing wrong? What is the proper way to edit or update changes to style.css?

Thanks.