To Edit a CSS you simply open it with notepad, make changes, and save it.
You can leave it open so you can get easy access to it when needed.
It is a good idea to save a copy of your CSS before making lots of major changes.
Note: If you are creating a new Style Sheet you must remember to save it with the '.css' extension
The best advice when making changes is to copy/paste css code that already works and make changes to it.
If you wish to point a page to a different Style Sheet you make the adjustment in the LINK command in the HEAD section near the top of the SOURCE code (use view source).