NAS325v2: Proper procedure to update firmware via USB stick?

bheem

ssue: After an unattended upgrade to 4.81 (most likely, as I didn't pay attention) via the web UI, I can't hookup to the volumes via smb anymore.
The system LED is constantly blinking green.

The NAS shows up on my router, DLNA seems to work (as I can see videos/pictures on the TV),
but I can't connect via smb or telnet as I used to.

I therefore decided to manually downgrade the firmware again.
I downloaded various ones for my model from firmware ftp://ftp2.zyxel.com/, but it is unclear
- what the directory structure on the USB stick should look like
- which USB port to use on the NAS
I tried with 
- unpacking zip in root dir and trying all three USB ports.
I never saw an orange blinking of the system LED, so something is not going as expected.

Last resort would be to reset of course, but I want to avoid reconfiguration if possible.
Anyone any idea?
TIA,

#NAS_Jan_2019