Have tried to upgrade several times. The download stopped several times, but did say it was complete. So I turned on the spark and controller and started the update. It failed. I shut all off and started over but it failed again and again. The spark is not working. What to do?????

Mine too. Rc no longer linked to bird.

Did you try using Assistant to do the upgrade?

I had to use my computer with DJI Assistant to update my 2nd and 3rd battery.  I also had to relink my RC after updating with the 1st battery.

Did you get any error message when it failed to upgrade? And please try to upgrade the drone via DJI Assistant 2:

I had the same problem... It took my five tries but it finally worked... Others have said use DJI Assistant software and connect drone to your computer.. that might be a last resort.

I was getting an error updating the firmware initially.  I ended up deleting the DJI GO 4 app from my phone and re-installing it from the Apple app store.  After doing that, the firmware update ran perfectly.