Hard-reset or a factory format will downgrade your Linux version firmware to the original OS version.
For example: if your charger is installed with 4.0.4 firmware, a hard-reset will downgrade the firmware to 4.0.
Please note, in case you are using a Windows version of the charger, a hard-reset is equal to a Factory Reset. It will not change the firmware version, but only set the configuration back to factory configurations. If you wish to downgrade a Windows version (for example: from 1.13.9 to 1.13.6) you simply install 1.13.6 via WebUI.
How to do a hard-reset / factory format:
reboot the charger via the service menu or the WebUI
once the chargers start rebooting, press and hold the service button
hold the service button, until “booting” shows up on the LCD screen of the charger