OneNote includes a small gallery of formatting styles that you can apply to a line of text with the click of a button.
Do the following:
Click anywhere within the text you want to format (for example, in a line of text that will become the heading of a paragraph).
On the Home tab, move the mouse pointer over the Styles gallery (Heading 1, Heading 2...) and then click the click the down-facing arrow that appears. On the menu that opens, click the text style you want to apply (for example, Heading 1).
If you change your mind, simply apply a new style while the text is still selected, and your previous formatting choice will be replaced.
Tip: To remove previously applied style formatting, select the text, and then click Format on the menu bar. Point to Styles, and then click Normal. Alternately, to immediately remove an applied style from selected text, press Shift-Control-N on your keyboard.