So, you're cruising along, writing your document, and you decide you'd like to modify one of the heading styles.
A fast way is to update the style based on actual formatting in your document. That is, if you want your Heading 2 to be indigo instead of black, select one of paragraphs that uses a Heading 2 in the document, and use the Font Color button to change the color. Then use the Update to Match Selection command (as shown in the picture) to update the style.
Everything that has this style updates to reflect the change.
Note When you first apply the new formatting to the selected text, Word interprets this as direct formatting and includes a new description in the task pane, such as "Heading 2 + Indigo." But once you update the existing style, this description disappears and the original style is modified.