Zyxel NSA325 Newbie question for update uBoot (1 reply)

First of all, sorry for the question but I don't understand the tutorial.

I have an old Zyxel NAS325v2 and want to use a new Debian on it. I have installed the latest original firmware from Zyxel.
I detached the NAS and only have the mainboard in front of me with the power cable, the network cable and the serial cable connected to the board.

From my PC (Linux) I have a serial connection via "screen" to the Zyxel mainboard. I interruppted the autostart an the command "version" gave me the following output:

U-Boot 1.1.4 (Jul 18 2013 - 10:47:29) Marvell version: 3.5.9

That is the uBoot version of the original firmware from Zyxel. I read the uBoot tutorial many times but I didn't know how to start.

I have downloaded the uboot.2016.05-tld-1.nsa325.bodhi.tar file. But what is the next step the commands described in the tutorial are not known in the serial connection.

I think I need a Linux on the NAS but that is the goal, I haven't had this for now.
Do I need a working Linux on an USB-Stick to update uBoot or has the original firmware from Zyxel the possibility to run the described commands for updating uBoot on the NAS?