Hi. Xmosis. Unless your paragraph is in a separate section, i.e., separated fore and aft with section breaks of the "Next page" type, what you are looking for is not margins but indents, left and right. With your cursor in the relevant paragraph, simply drag your indents in from the sides on the top ruler bar. If the bar is not showing, hold down the Alt key while pressing first V, then R (or go to View/Rulers and select Horizontal).

Best,
Ulodesk