At the time of writing (October 2019) the latest BIOS is 3.0U.
Note: There are two variants of this, one for the ThinOS (Linux) versions of the D-class, the other for the Windows versions. The ThinOS variant is available as a direct download from the Dell/Wyse support site. The Windows variant is embedded in the WES7/WES8 downloads which are ~9GB in size as those downloads include the full operating system, not just the BIOS file.
The BIOSs are slightly different. The Windows variant includes elements of the Dell/Wyse Windows Licence. If your D-class is currently running WES7 or WES8 (or did in the past) and you wish to continue using it as such then you need to download the full package and use the standard Wyse tools to update/re-install the OS.
If you have no interest in using the standard Dell Wyse Windows distribution then by all means do the 2MB download and use flashrom to update the BIOS.
The approach that was originally described here applies equally to the Z series. Rather than repeating the description for each thin client it has moved to a common location here.
Any comments? email me. Added May 2019 Last update October 2019