This is the download for DJI Assitant 2 the latest version is 1.1.6,
under software there is a exe file for PC's and one for IMac, just download and run it to install
DJI Phantom 4 Pro – Specs, FAQ, Tutorials and Downloads
Then connect the P4 via usb to the PC, then turn on the P4 and open the DJI Assistant app, you'll have to sign in with your DJI account, when connected you will see a Phantom 4 icon, hit this icon to enter the main menu which will have the firmware update showing.
Make sure you have a full battery before running the firmware upgrade