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This is something that you need to do using the payment processor system, if you're referring to this form: http://www.jotform.us/form/41697894856175 then you would need to do this using your Authorize.net account.
You can read more about this process here: https://www.authorize.net/support/CP/helpfiles/Search/Transaction_Detail/Refund_a_Transaction.htm
I hope this answers your question.