Using Styles in Word opens up a whole new set of tools you never had before. Change styles, however, can be tedious and cumbersome, unless you know this trick:
After formatting some text to a style, using Heading 1, make the formatting change directly in the content. Then right-click on top of the changed text and select Styles, Update Heading 1 Style to Match Selection. Now all Heading 1 style formatted text will inherit the new change. It might be a good idea to save the current theme to avoid having to re-create the style each time.