Styles applied from the 'styles' drop-down list can (as Dani says) be removed by selecting the styled area, and clicking the same stye in the drop-down list again. It should have a little arrow to the left of it to show it has previously been applied.
The easy way to remove styles from formatted text - by selecting individual styled areas, whole sections containing multiple styles, or indeed the whole editable area text, is to select the selections, areas or whole page, and click the 'Remove Formatting' icon in the toolbar.
See attached image.