There is no such thing. ST.23.16.50 was the only firmware ever made for the DCM235 so that's what you've got, and that's what you'll keep.
Lemme guess...you're trying to use it via USB, right? And every so often your connection simply hangs, or else Windows XP freezes. Sound familiar? Join the club. There's a zillion DCM235 boxes out there in the world and they all do the same thing. The average user doesn't stress the modem hard enough and keep it on as long as us geeks so most people don't think there's anything wrong with the DCM series but I'll never buy another again.
Connect via the Ethernet port, and you'll have less problems.