Find the form you would like to edit and click on the blue "Edit" link under the "Action" column.

On the next page, Step 2: Form Options, click the orange "Save and Continue" button. This will take you to the Step 3: Select Database page. Here, in the last box, you will see a list of your data fields. You can change the order of them by clicking the field name, and while holding down the mouse, drag the field name to the place where you would like the field to be.

Click the "Next >>" button and click through the rest of the pages until you get to the screen with the "Save" button. Click it and you are finished.