To add a subhead: Type in the wording for the subhead. Click anywhere in that line. In the menu box, look for the dropdown one that says Paragraph. The Heading ones control the subhead look, with Heading 1 being the largest and Heading 3 being the smallest. To match to something that’s already on the page, just click on that text and it will show the heading style being used. You can then use that same style for the one you’re creating.
To undo what you just screwed up: Click on the icon that looks like an arrow curling back to the left.