Can anybody help me as I tried to Update my Phantom 4 to the latest firmware update using the DJI Go4 app.   The firmware is stuck at 99% and I then googled the Problem and DJI Assistant 2 was suggested. I tried this but it would not recognise the Aircraft.   Can any of you good folk help me to solve this problem.

Mine done the same
I just unplugged it and started again seemed to have worked
I think the correct way is to update the rc unit first then aircraft but mine didn't have that option
I'm sure someone else will reply with the exact method to update

Check information in DJI Go 4 'General Settings'/'About' menu, update may have completed OK but not being reported correctly.

Check the current firmware first, like Aardvark suggested, since you have been reached at 99%, the update may have completed. If it haven't, you can try to re-install the APP, re-start the drone, then re-upgrade it again.