In the left Navigation Menu, go to Forms -> Manage Forms (Fig 1)

Fig 1

Click the "Edit" link for the form you want to edit (Fig 2)

Fig 2

Click the "Continue" button to advance to the next page. You can drag the Data Fields from the Toolbox column on the Left into the Form Layout column in the middle. In the Form Layout column, you can also drag the fields around to change their order (Fig 3).  

Fig 3

When you're finished, scroll to the bottom and click on the "Save" button to save your changes (Fig 4).

Fig 4