Pick the json field and replace the existing property value in the payload using the script

import com.sap.gateway.ip.core.customdev.util.Message; import java.util.HashMap;

import java.text.SimpleDateFormat;

import java.util.TimeZone;

import org.joda.time.*;

import groovy.json.*


def Message processData(Message message)

{

def ExtId1 = message.getProperty("ExternalId").toString().trim();

def body = message.getBody(java.lang.String) as String;

def jsonSlurper = new JsonSlurper();

def jsonDataObject = jsonSlurper.parseText(body);

jsonDataObject.header.externalid = ExtId1

message.setBody(new JsonBuilder(jsonDataObject).toPrettyString()) return message

}