In Node-RED use lora-in to receive packets from the end-device, use lora-out to send back to end-device.
Add lora-in and lora-out nodes to flow with function node between them.
Put this code in the function node to disable ack for the downlink packet:
msg.ack=false;
return msg;If you want to change the payload.
msg.ack=false;
msg.payload = "new data";
return msg;A buffer could also be used.
msg.payload = new Buffer([0x01, 0x02, 0x03]);