To edit an online form select the appropriate form under Online Forms, then Forms Builder and select edit from the payment form you wish to change.
From there, you can add any fields you would like by dragging and dropping any field under Fields to the right-hand side of the screen. You can delete fields by clicking the trashcan icon below the field.
After dragging your desired field to your form, the Field Settings section will show your options for the new field. Start by setting a Field Label, then check if this should be required and if it will display in the emailed receipts.
You can also edit existing fields by clicking on the blue pencil icon that appears next to the field. This will allow you to change the name of the field, make it required or allow it on your receipts.