I have no gimbal/camera (crashed/destroid) Can I still upgrade the aircraft?
If so, how to do it?

Interesting question but the short answer for us mere mortals is going to be no.  I suspect the wizards at DJI might have a secret decoder ring to do it thru the aircraft USB port.  

Shifting gears for a moment, what would be the point of bothering with an update since you don't have a camera and gimbal?

Nope, it is not possible to update without the gimbal