Tried updating with the SkyAtlas app, it failed multiple times: the AM5 wifi would always briefly disconnect, which exits the AM Management menu and the FW never gets updated.
Then tried with the new ASIAIR app (2.3 beta) upgrade feature: gets stuck forever on "Updating ASIMount Firmware" at 0% progress.
So I went back to the SkyAtlas app, rebooted the AM5 mount, tried again. The wifi connection to the AM5 keeps disconnecting; I just keep trying and after a while when the mount wifi is up again, the Firmware menu states that the mount is now running 1.6.3.
So it looks like it ended up working but for some reason the upgrade process is very chaotic and not user friendly. I still don't know if it was a bug, a busy upgrade server, something else - or, just that the upgrade process is bad at communicating information to the user...