To edit an online form select the appropriate form under Forms Builder and select edit from the drop-down menu that appears.
From there, you can add/delete any fields you would like by dragging and dropping any field under Field Settings to and from the left-hand side of the screen or the right-hand side of the screen.
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.