Firmware update require Wi-Fi connection. If you connect to unit hot spot, browser automatically will open captive portal, where you could upload new firmware file. Firmware file should be unarchived before upload to DM Transmission. When unit connected to other wi-fi network as client, you could update it too, but you have to manually open web interface by URL: http://[dm-transmission-ip-address] - where IP address assigned by network router.
After firmware file sucessfully written, power cycle controller and wait at least 10 seconds before next power-off. If unit rebooted twice in a row - previous firmware will returned back. This mechanism check that new firmware working correct at least 10 seconds after boot, and then mark it as valid