Modifying

There are a few ways you can update a style in your document.
Using the Styles and Formatting task pane
Using the Style Dialog Box

Using the Styles and Formatting task pane

Once of the quickest is to use the Styles and Formatting task pane.
Select the text you want to change and apply the necessary manual formatting
Using the Styles and Formatting task pane click Heading 1 and select "Modify" from the drop-down list.

Using the Style Dialog Box

Important

As soon as you change a style in a document that style is automatically updated throughout the document.