How to Update Existed Payload Value in Mule Dataweave

You need to pass the field name as String in the update function. If you change your expression to referralPayload update('placeId') with then your dw will work.

To use the complicated conditional logic while updating I would recommend using the update operator rather than using dw::util::Values::update function because it provides a more clear and readable syntax

%dw 2.0
output application/json
var referralPayload = {"data":1,"placeId":"BSSE","topValue":1000}
---
referralPayload  update {
        case .placeId ->  if(!isEmpty($)) referralPayload.topValue else $
}

Below are the two approaches based on your requirement:

1. Using the "update" operator:

%dw 2.0
output application/json
var referralPayload = {"data":1,"placeId":"BSSE","topValue":1000}
---
referralPayload update {
        case .placeId ->  if(!isEmpty($)) referralPayload.topValue else $
}

2. Using the "update" function:

%dw 2.0
output application/json
import update from dw::util::Values
var referralPayload = {"data":1,"placeId":"BSSE","topValue":1000}
---
referralPayload update "placeId" with (if(!isEmpty(referralPayload.placeId)) referralPayload.topValue else referralPayload.placeId)

Approach 1 is more optimized:

1. importing a library is always a costlier process
2. use of "update" operator can be used as a switch case that is more efficient in case of multiple updates operations