In TurboTax Online all information has to be entered through the question-and-answer interview. You cannot access the forms directly. Is the comment that you want to change something that you entered, or did TurboTax generate it? If you entered it, go back to where you entered it and change it. If TurboTax generated it, you cannot change it.
What is the specific supplemental statement that you want to change? If it's in a state tax return, what state is it?