I have successfully downloaded the firmware but cannot upgrade. Anyone else having this problem?

Did you download via dji page or via app and what version of the firm ware is it?

Via the app.

Did you download via household WIFI or mobile data line?  IF you did it via household WIFI then you need to switch between the household WIFI and the Phantoms WIFI,so the FW-file can be uploaded to the RC/AC of the Phantoms system. And during upgrade the FW the RC and AC must be switched on.

I would recommend you to check this video and try again.