string jsonBody='{"payload":[{"YearlylCostPercentage":"4.6","TotalRepayment":"120000","PropertyMarketValue":"100000","productname":"ABN Standard+","PaymentFrequency":"Monthly","NumberOfTerms":"60","NotaryName":"Notary","NotaryEmail":"raju.sen@yahoo.com","NotaryContactNumber":"0649991729","NotaryAdressLine2":"2563 CM The Hague","NotaryAdressLine1":"Valkenboslaan 72-F","nominalInterestRate":"4.5","MonthlyPayment":"600","MonthlyInterestAmount":"300","loanUtilizationDate":"12-1-2019","loanPurpose":"Loan","loanMaturityDate":"12-2-2019","LoanDuration":"5","loanAmount":"100000","KadasterLocation":"Amsterdam","InterestFixationProvision":"1.0","HandlingCost":"900","FixedInterestyear":"2.0","firstPaymentDate":"12-2-2019","FirstMonthlyPayment":"700","effectiveInterestRate":"4.7","DepartmentName":"Dev","CustomerDetails":[{"CustomerSalutation":"Geen titulatuur gewenst","CustomerName":"Eqaavq","AddressLine2":"1221 NH MDFMGDAKSV","AddressLine1":"Wdr Veuniztapbmr 345"},{   "CustomerSalutation":"Mevrouw","CustomerName":"Eqaavq-col Fpcysfptp","AddressLine2":"1221 NH MDFMGDAKSV","AddressLine1":"Wdr Veuniztapbmr 345"}],"city":"Amsterdam","BorrowedRatio":"1.4","AdvisorName":"Raju Sen","AdvisorContact":"0649991729","AdviseCost":"1000"}]}';
Map<String, Object> jsonParseSource = (Map<String, Object>)JSON.deserializeUntyped(jsonBody);
system.debug('Sai tets '+jsonParseSource);
List<Map<String, String>> payload = new List<Map<String, String>>();
List<Object> payLoadList=(List<Object>) jsonParseSource.get('payload');
Object PayloadObj;
if(payLoadList.size()>0)
{
    PayloadObj=payLoadList[0];
}
String customerDetailString='';
Map<String, Object> payloadBody = (Map<String, Object>)PayloadObj;
system.debug('Sai tets '+payloadBody);
for(String key: payloadBody.keyset()) {
    system.debug('Key '+key+ ' Value '+payloadBody.get(key));
    if(key == 'CustomerDetails')
    {
        List<Map<String,String>> tempMap= new List<Map<String,String>>();
        List<Object> tempList=(List<Object>)payloadBody.get(key);
        for(Object obj : tempList)
        {
            Map<String, Object> tempObj = (Map<String, Object>)obj;
            for(String tempKey: tempObj.keyset())
            {
                tempMap.add(new Map<String, String> { 'name' => tempKey, 'value' => (String)tempObj.get(tempKey) });
                system.debug('List Key '+tempKey+ ' List Value '+tempObj.get(tempKey));
            }
        }
        system.debug('tempMap'+tempMap);
        customerDetailString=JSON.serialize(new Map<String, List<Map<String, String>>> {
            key => tempMap
                });
                system.debug('customerDetailString'+customerDetailString);
    }
    else
    {
        payload.add(new Map<String, String> { 'name' => key, 'value' => (String)payloadBody.get(key) });
    }
}
String jsonResult = JSON.serialize(new Map<String, List<Map<String, String>>> {
    'payload' => payload
        });
String custpomerObjTemp=customerDetailString.substringAfter(':');
system.debug(custpomerObjTemp);
custpomerObjTemp=custpomerObjTemp.Left(custpomerObjTemp.Length() - 1);
system.debug('Reult '+ custpomerObjTemp);
String finalResult=jsonResult.Left(jsonResult.Length() - 2);
finalResult=finalResult+',{"value":'+custpomerObjTemp+',"name":"CustomerDetails"}]}';
system.debug(finalResult);