I was answering questions about my dependents and I checked that my son supported himself for more then half the year on accident because I thought it meant me

Sure, just click on My Info on the left side of the online program screen and edit the dependent information for him to change your answer.

Yes, go back to the dependents section - it is in the My Info section (see left side menu).   Click on edit next to your dependent and carefully/slowly page through the questionnaire.    You can change your answer when you get to that screen.