JSON File - Java: editing/updating fields values

I have some JSONObject(s) in my workflow, and the same JSONObjects are stored by writting them into a json file.

I would like an efficient way to update the json file, only fields where is needed, with the content of newer JSONObjects instances.

Eg:

On file i have

ObjectOnFile = {key1:val1, key2:val2,...}

In memory i have

ObjectInMemory = {key1:val1_newer, key2:val2_newer,...}

The update will be like:

 if (ObjectInMemory.get(key1) != ObjectOnFile.get(key1))
        "ObjectOnFile.set(key1,val1)" <--- how to?

In general I would like to update the value of every key where its content is newer (different).

Actually my code is:

Sting key = "key1"; //whatever
JSONObject jo = new JSONObject("{key1:"val1", key2:"val2"}");
JSONObject root = mapper.readValue(new File(json_file), JSONObject.class);
JSONObject val_newer = jo.getJSONObject(key);
JSONObject val_older = root.getJSObject(key);
if(!val_newer.equals(val_older)){
   root.put(key,val_newer);
/*write back root to the json file...how? */
}