You want your headings to be in a heading style (custom or built-in) and your body text to be in the Body Text or Normal or another custom body style. Then you can simply modify the needed style. It is not too late to apply a heading style to your headings. There are keyboard shortcuts built in for Headings 1-3. Your document is currently likely all in the normal style. This is not a good practice.

• Importance of Styles in Word
• Why Use Word's Built-In Heading Styles? by Shauna Kelly

To modify a style that is in the Quick Styles Gallery, right click on it in that Gallery and choose "Modify." For others, launch the Styles Pane, using the dialog launcher at the bottom right corner of the Quick Styles Gallery.




I modified your question title to "headings" rather than "headers." This distinction is important for others trying to find the question and for you when trying to find answers about Word. What is the difference between headings and headers?

If you use Word much at all, I recommend going through the short tutorials here: Basic Concepts of Microsoft Word - from Shauna Kelly Doing so will save you a lot of time and frustration.