The beauty of styles is that you can change their definition at any time. Changing a style definition means that all the text formatted with that style changes to match the new style definition. This has been a staple feature of all page layout programs since the Stone Age. Yet it still blows me away.
Styles allow you unprecedented flexibility in experimenting with the appearance of your document. A few simple clicks and you can totally transform the appearance of your pages. It's a beautiful thing.
There are several ways to edit a style, but to my mind the best (and safest) is to Control+Click (Right Mouse+Click) on the style name in the appropriate Styles palette. That way, there's no danger of inadvertently applying the style where you don't want it.
Once you're in the Styles Options box, make as many or as few changes as you want, click OK, and, as they say in cockney gangster movies, "Bob's your uncle."