You have not mentioned the exam you are inquiring about which makes it difficult to help you, but if you are not supposed to pay at this stage or have not yet paid, then I think filling a new form shouldn't be a problem. If that doesn't work and the system says that this information has been already filled, then you can contact the respective authorities to raise the query for correction. Had you been clear about the exam, I could have given you a better way out of this but still hope this works for you