On the left side of your screen, go to My Info.
On the "Personal Info Summary", click the pencil (for Edit) to the right of your name.
On the next screen "Your Personal Info", look down to the subheading "A few other things we need to know".
You can check Yes or No to being a dependent here.
When you do this, be sure to continue through the interview until you get back to "Personal Info Summary".