How can I change the styles in onenote

Editing files under the Office installation directory is not a workable way to redefine OneNote's styles. Those resources are application components, may be protected, and can be replaced by an Office update. OneNote does not store the built-in Heading, Citation, and Quote definitions as a supported user-editable style sheet.

For university notes, create one page with your preferred title, headings, body font, spacing, tables, and callout layout. In desktop OneNote, save that page as a template and set it as the default for the section where appropriate. Duplicate the template for each lecture or assignment, and use Format Painter for formatting that must be repeated inside a page. You can also set the default font under File > Options > General so newly typed body text starts closer to the required format.

If the final submission must follow formal styles, keep OneNote as the capture and organization tool, then move the finished content to Word and apply the university's Word style set there. Word styles carry semantic heading levels and can generate navigation and a table of contents; manually formatted OneNote text does not provide the same document structure.

If you’re trying to change the look of headings or text consistently, it’s worth checking OneNote’s built-in styles and formatting options first. Some versions of OneNote have different style controls, so the exact options can also depend on whether you’re using the desktop or web app.

You cannot create fully custom styles in OneNote like you can in Word. OneNote only offers a fixed set of built‑in styles (Headings, Page Title, Citation, Quote, etc.), and while you can apply formatting manually, you cannot redefine or save new styles. You may only consider using the Format Painter to copy formatting across text.

https://support.microsoft.com/en-us/onenote/customize-a-style-in-onenote-for-windows