You are asking for firmware, do you know which version it is running.
I did have one but am a bit hazy on details however you need the correct app to connect and in theory you should still be able to download most recent firmware.
The app and controller both need to be on versions that talk to each other, if it is working without updating I would avoid it now as they won't be offering support most likely.
You could also try litchi as suggested above.
https://www.dji.com/uk/downloads/djiapp/dji-go-3