If you want to use the GUI based updater, do NOT click on Download Firmware — the updater program hasn’t been changed for a long time and the original download link is no longer available. Instead, the firmware files are together with the GUI program. So all you need to do is to Upload. Again, do NOT click ‘Download Firmware’.
Please also note that the GUI program only has firmware up to 2.1.7. If you want to update to 2.1.9 you should follow the instructions and use the avrdude program to update.
The GUI program is just a wrapper for the avrdude command. But as operating system updates often render previously compiled programs obsolete, we don’t have enough resources to keep updating the GUI program and make sure it works with the current operating systems. So we stopped maintaining the GUI and instead recommend you to use avrdude, which does not have operating system compatibility issues.