There has only ever been one software change on the 840C. This made no change whatsoever to the performance or functionality of the 840C and was only to address a problem caused by the Apple Airport Express if the user was wishing to use the 840C as a DAC with the Airport Express.
Basically, the AE has a bad optical output that occasionally causes a high pitched sound, and as Apple did nothing about this, we were able to implement a software change on the 840C that mitigated the effects of the AE's optical output.
The original firmware version was 067, and the new one is 069, which was worked into production in October 2009.
So, your dealer will be able to arrange for this update if you require it, but note that it is only of any use if you intend to use the 840C as a DAC specifically in conjunction with the Apple AE.