that depends on what you want to do. If you want to change certain aspects of the existing style sheet, then modify what is there. If you are adding more elements, you can either add them to the theme's style sheet of create anthother and link it in in the top 'head' tag. kinda like this: <link rel="stylesheet" type="text/css" href="http://www.yoursite.com/style.css" /> Code (markup): Make sure to use the full URL as this will make it easier for the page to find. Note: if you have the same names for elements in each style sheet, the browser will use the last one it reads. Meaning, if you install the above link to the style sheet below the one that is there, it will read that one last and use the elements from that.
Do it from the backend. Go into admin, select style, then select theme, and click edit. Here you can add to the existing css, or replace what ever you want to.