On our web site we keep a zip file that contains the latest version of all firmware files for the products we make. You can find it on the Firmware page, it's named All_Hosteng_Firmware_Files.zip.
Once you've downloaded that file, the trick is unzipping the contents into the correct folder, and that's determined by which programming software you're using:
if you've installed Do-more Designer v2.2 then unzip the file to \users\public\"public documents"
if you're still running any version of Do-more Designer previous to v2.2 OR you're using DirectSOFT & NetEdit, the correct folder is most likely the root folder of the C:, that is "C:\".