If you are running a PLi image, you don't have to use dreamup, and you don't have to use a serial cable.
Select the 'software update' menu item, and your box will reboot in bootloader mode.
From there on, you can use the same instructions for flashing a 7020
http://www.dream-multimedia-tv.de/dm...ng_browser.php
The 7020 uses the same bootloader as your 600.
But there is a difference; the 600 has no buttons, to signal the bootloader to accept new firmware.
That's why we implemented this 'software update' thing in the menu, to signal the bootloader through some other means.
Now, if you really can't use dreamup (because you don't have a windows PC for instance), you could try the following:
login with telnet
mount /boot -o remount,rw
rm /boot/*
reboot
Now you've got a box which won't boot anymore, so the bootloader will accept new firmware.
Only do this if you have a way to tell which IP your box got, after you reboot.
Because you won't be able to use your box again, untill you flashed an nfi, using the webinterface of the bootloader.
Flashing goes really fast, and is really easy, but before you attempt this, make sure your network works, you have a working DHCP server in your network, and you know which IP your box will get, because that's where you have to connect your browser to, in order to flash the new firmware.