If you just want a different theme, you didn’t need the CSS upgrade. Just go to Design – Themes then select the theme you want and activate it.
If you want to amend a theme and you have the CSS upgrade, go to Design – Edit CSS and you will see the CSS editor screen. Either paste in a whole stylesheet and check ‘Start from scratch’ or paste in CSS overrides and check ‘Add to existing Stylesheet’. There’s an option that lets you view the existing stylesheet and a really useful preview function.