I am only guessing that they are constantly updating, but the changes might just be with each new version of the machine. There was a long discussion on this on apneaboard and I believe the consensus was you can't update your machine as you can update software on a computer, or GPS.
I am like Mary and think they are constantly updating, but that it all affects new machines and not ones that are already in use or produced.
Although it would be nice if you could some how download and update the software on our machines, I doubt if it will happen any time soon. The manufacturers don't have any incentive to provide that to the consumers probably.