Below is a link to the current firmware update file for the Duet System - M2T transmitter and M2R receiver. 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 know you are updating either the M2T or M2R and automatically choose the right firmware images from the file.