Overwrite JSON object without replacing the file completely Android Studio

Basically I am trying to overwrite the following JSON data

{
    "name" : "John Doe",
    "gender" : "male",
    "age" : "21"
}

My goal is to replace only the age. So I am using FileOutputStream like below

JSONObject object = new JSONObject();
try {
    object.put("age", "40");

} catch (JSONException e) {
    //do smthng
}

String textToSave = object.toString();

try {
    FileOutputStream fileOutputStream = openFileOutput("credentials.txt", MODE_PRIVATE);
    fileOutputStream.write(textToSave.getBytes());
    fileOutputStream.close();
    intentActivity(MainMenu.class);

} catch (FileNotFoundException e) {
    //do smthng
} catch (IOException e) {
    //do smhtng
}

But with the above code, I realized it completely deletes the existing credentials.txt which means I lost the name and gender value. Is there anyway just to replace the age? Thank you

I've found the answer to this. Basically I need to read the existing JSON file and then replace the existing JSON object value

private void writeFile(String age) {
    try {
        FileInputStream fileInputStream = openFileInput("credentials.json");
        InputStreamReader isr = new InputStreamReader(fileInputStream);
        BufferedReader br = new BufferedReader(isr);

        StringBuilder jsonStringBuilder = new StringBuilder();
        String line;
        while ((line = br.readLine()) != null) {
            jsonStringBuilder.append(line);
        }
        br.close();

        String jsonString = jsonStringBuilder.toString();
        JsonParser jsonParser = new JsonParser();
        JsonObject jsonObject = jsonParser.parse(jsonString).getAsJsonObject();

        jsonObject.addProperty("age", age);

        FileOutputStream fileOutputStream = openFileOutput("credentials.json", MODE_PRIVATE);
        OutputStreamWriter osw = new OutputStreamWriter(fileOutputStream);
        BufferedWriter bw = new BufferedWriter(osw);

        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        String updatedJsonString = gson.toJson(jsonObject);

        bw.write(updatedJsonString);
        bw.flush();
        bw.close();

    } catch (FileNotFoundException e) {
        intentActivity(Login.class);
    } catch (IOException e) {
        intentActivity(Login.class);
    }
}