John,
All you would need to do is to copy it into your project
folder, overwriting the old file. I haven't used RH6 but I can't
imagine it would be any different. You could do this from Windows
Explorer. The usual caveats apply - i.e. make a back up of your
current css. Since it has the same name, you wouldn't need to do
anything to your topics. Recompile and voila - new style.
John