For the suggestion on DJI assistance 2, just note that the date of the app is still 2017-12-29.
It appears to me that the desktop app has not been updated yet for the Mavic Air. (If that is even necessary, I don't know)
The video from DJI that shows the new firmware update to the Mavic Air being done from the Go 4 app.
[Edit]
Oops. I made a mistake on that.
On the Mavic Air's FAQ's, there's a question on update failures. It recommends then using the Assistant 2. Hmm. :
5.What factors could lead to an update failure?
a. Low-battery level. Make sure the battery level is higher than 50%.
b. Powering off the Mavic Air or the remote controller during the update. If this happens, please restart the Mavic Air and the remote controller and begin the update again.
c. Internet disconnection when downloading firmware.
d. If update failure persists, or the remote controller does not link with the aircraft, there may be firmware inconsistencies between devices. In this case, use DJI Assistant 2 on your computer to update your aircraft and the remote controller to the latest versions separately.