I mistakenly said someone else can claim me as a dependant. This is not true, but I can't seem to get back to the section to change the field.

Have you already filed your return?

no, I'm just starting.  but every time I try to go "back" I can't get to that section.

 You will log in to your TurboTax Online account, and click Take me to my return. Next, you will click on Personal Info (located toward the top of your screen) and locate your name. Click to edit beside your name. On the following screen near the bottom, you will find the area to click that you cannot be claimed as a dependent by someone else. I will attach a screenshot of that area, so you are better able to locate it.