Depending on your hosting provider, you can normally use your control panel to some extent to edit files within your hosting environment. Additionally, you can likely find a plugin that allows you to edit files from within wordpress with a little search. You'll need to be sure the permissions for the file(s) you want to edit have been set correctly though.

Lol...

Go to Wordpress Admin -> Appearance -> Editor -> Styles and choose style.css (or whatever you need)