Go to the course builder and to the text you would like to edit.
Click on the text section so you enter text-editing mode. A toolbar will appear when you are in editing mode.
Select all text you want to reset the styling for.
Click on the reset styling icon in the toolbar. It looks like a T with a smaller x after it. The icon looks like this:
Finally, to remove all structural formatting, click on the format menu to the right of the toolbar. It usually says "Normal" or "Header 1". In the drop-down menu select "Normal" while still having all text selected.
Tip: you can also import the text without formation by clicking CTRL+SHIFT+V.