My understanding is that the gearbox software is updated using genuine STAR. You will need to go and see your local MB dealer, or an indie with genuine STAR and active subscription.
Said that, it is worth confirming first that the issue is not caused by other factors, e.g. when was the ATF last changed?
The reason for this is that I was advised that the software update sometimes fails and in such cases a new transmission is needed - i.e. not just new TCU. Perhaps not an issue if the car is still under warranty, but not something you would want to foot the bill for if it isn't.
I don't know for fact how often this might actually happen, but it was enough to put me off doing it...
