Below is a link to the current firmware update file for the D² Digital System - DSQD receiver and the DBu, DBu-LEMO and DHu transmitters. This is a zip file, after downloading extract it into a temporary folder. Use the Wireless Designer™ program to update firmware. Instructions are found here.
Note: This is a link to the Wireless Designer firmware update file, which bundles the latest firmware for multiple products. Wireless Designer will automatically choose the right firmware images from the file.