Does anyone know a way to re-install the firmware update on the Osmo Action?

I'm not sure if this will work or not.  But you can load the firmware onto the Action by downloading the file from the DJI support site and placing it (unzipped, I think) on an sd-card.  Then start the camera.  BUT I don't know if there's a check to see if the firmware on the card is new or different.  So if there's no change, it might not reload it.

I don't know if the Osmo app on your phone/tablet has a reload function.  I've not looked at it since the initial activation and upgrade.

Just checked, my mimo app on my android phone does not have a reload mode.
Maybe it's possible with a laptop but i don't have access to one right now.

How about factory reset of the device?

Hi, thanks for the inquiry. Apparently, re-installing the firmware can't be done with our Osmo Action, however, we're currently checking this inquiry to see if this would be possible. Thank you for your support.