Itr processing after correction in challan details

An individual made mistake while paying advance tax/self assessment tax online. Instead of using Major head as 21 he used 24. The ITR was processed and demand was determined because Adv tax/ SA tax was not considered. Unfortunately, the so called tax expert was not able to find this discrepancy and in rectification he repeated the same data without any correction. CPC transferred the rectification rights to AST. When I checked, I could note the discrepancy. The error has been corrected by making written request to AO. Fortunately, error in challan is corrected now. The question now is how to get refund? Is there any prescribed format for seeking refund in such cases which are pending with AO/AST?