Unable to change the attribute value present in Json using JSONObject

I need your help here as my requirement is to have the value for one of the attributes called transactionReferenceNumber which comes under JSON path Request.eventDetails.message section.

Of course, with little knowledge on the JSON path traversing I was able to traverse till Request.eventDetails.message using JSONObject as shown in below snippet code, but unable to traverse in message section and get or change the value for transactionReferenceNumber attribute or any other attribute.

String filepath = System.getProperty("user.dir") +
    "/src/test/resources/Json_Files/Payment_Events_Api_PayLoad.json";
String jsonContents = new String((Files.readAllBytes(Paths.get(filepath))));
JSONObject jsonObject = new JSONObject(jsonContents);
			
String Message = jsonObject.getJSONObject("request")
    .getJSONObject("eventDetails")
    .get("message")
    .toString();

Below is the JSON for your reference:

{
    "metadata": {
        "requestId": "e1d0db1f-ac7d-4011-b34e-c063b6d638a6",
        "signature": "ZwcQa6YozllV1mLWPrZFiacSguRoEibSgmb97UvTrWgvuY5I76eWuAg==",
        "signatureAlgorithm": "SH443222wesd"
    },
    "request": {
        "eventId": "e43e2222-2c91-47d8-8c03-9a5f52db15a2",
        "eventCode": "EADFE",
        "eventCategory": "PAYMENT_STATUS",
        "eventType": "TRANSACTION_STATUS",
        "eventDetails": {
            "paymentProduct": "CXC",
            "message": "{ \"achCompanyId\": \"3131231\", \"companyName\": \"ABC Company\", \"transactionReferenceNumber\": \"4333\", \"remittanceInfo\": \"Paroll Processing\", \"processingDate\": \"2022-01-12\", \"payeeFirstname\": \"John\", \"payeeIDType\": \"email\", \"payeeEmail\": \"abc.Doe@gmail.com\", \"payeeMobile\": \"\", \"paymentType\": \"STANDARD\", \"paymentAmount($)\": 600.50, \"numberOfHoldDays\": 5, \"currentPaymentStatus\": \"PENDING\"}",
            "messageFormat": "CUSTOM.CXC.JSON"
        },
        "timeStamp": "2020-07-21T14:53:54-0400"
    }
}

Please, request in all to help me on this issue and also suggest the possible way to tackle this problem.

Thanks in advance.

The "message" value in your JSON is a string containing another JSON object, not a direct JSON object, meaning that you cannot traverse it directly using getJSONObject() or similar methods. What you need to do is to parse the "message" as a separate JSON object, and only then you can modify its values:

String filepath = System.getProperty("user.dir") +
    "/src/test/resources/Json_Files/Payment_Events_Api_PayLoad.json";
String jsonContents = new String((Files.readAllBytes(Paths.get(filepath))));
JSONObject jsonObject = new JSONObject(jsonContents);
            
String messageStr = jsonObject.getJSONObject("request")
    .getJSONObject("eventDetails")
    .getString("message");

// get message separately
JSONObject messageJson = new JSONObject(messageStr);

// modify your value
messageJson.put("transactionReferenceNumber", "NewValue");

// replace the old message string with the modified one
jsonObject.getJSONObject("request")
    .getJSONObject("eventDetails")
    .put("message", messageJson.toString());