If you need to make changes to a Paragraph Style, you could double-click on the style listing in the Paragraph Styles palette and then make the changes in the multi-tabbed dialog that opens. You can even check a preview box in the lower left hand corner of the window to see each of the changes update.
But there is a way that seems a lot more intuitive. You simply reformat one paragraph that has the Paragraph Style applied to it. Then, with the text cursor anywhere in the reformatted text, go under the Options menu of the Paragraph Styles palette and select Redefine Style. This applies your formatting changes in this one paragraph to all paragraphs that have the style applied.
Tip provided by Jeff Witchel, Certified Adobe® Training Provider.