You can try this method stringByReplacingPercentEscapesUsingEncoding:NSStringEncodingConversionAllowLossy]
Taken from this question urldecode in objective-c //
NSString *param=@"%7B%22paymentMode%22:%22%22,%22transactionId%22:%2231674%22,%22pgRespCode%22:%223%22,%22TxMsg%22:%22Canceled%20by%20user%22,%22authIdCode%22:%22%22,%22currency%22:%22INR%22,%22amount%22:%221.00%22,%22addressStreet1%22:%22Sesame%20street%22,%22addressStreet2%22:%22%22,%22isCOD%22:%22%22,%22loadStatus%22:%22fail%22,%22TxId%22:%22123456%22,%22addressCountry%22:%22India%22,%22firstName%22:%22Ankur%22,%22TxGateway%22:%22%22,%22signature%22:%2245558eb93513aa7a4f2fba24e0ba577b26eb5f40%22,%22addressState%22:%22Pune%22,%22lastName%22:%22Arya%22,%22addressCity%22:%22%22,%22TxRefNo%22:%22CTX1307151506338704178%22,%22loadAmount%22:%221.00%20INR%22,%22pgTxnNo%22:%22CTX1307151506338704178%22,%22TxStatus%22:%22CANCELED%22,%22email%22:%22daredevil.suyash@gmail.com%22,%22issuerRefNo%22:%22%22,%22mobileNo%22:%229900414420%22,%22addressZip%22:%22411045%22%7D";
NSString *newParam = [param stringByReplacingPercentEscapesUsingEncoding:NSStringEncodingConversionAllowLossy];
NSLog(@"%@",newParam);