Is this a FormsCentral PDF form or web HTML form?
You will need Adobe Acrobat to edit the PDF Form. Please try the following:
1. Refer to the FAQ http://forums.adobe.com/docs/DOC-3661 to edit the FormsCentral form in Acrobat
2. In Tools, select Forms / Edit
3. Double click on the desired field, you will see the field properties window. Here select Appearance tab and you should be able to change the border settings.
Hope this helps,