A style sheet is just a text file with a special extension which is CSS. This extension tells the browser that this file contains rules on how items on the webpage should be displayed.
You can edit a style sheet several ways. If you know the syntax you could just open it in notepad and edit away.
There is software available that guides you the process, and also lets you see you changes as you make them. The best known CSS editor is TopStyle.
TopStyle has pro (paid) and a lite (free) versions. Information and price can be found Here.
Lots of other editors available also, just do a google search and find the one that is right for you.