To edit a form go to Dashboard > Online Registrations > click the edit (pencil) icon next to the form name.
This will take you to the form editing page. Here you can change the details of the form, and add/remove sections and questions, add an end date on the form.
An outline of how to create sections and questions can be found here:
Click Save Form at the bottom of the page to save your changes, or alternatively click Cancel to cancel all current changes.