What you need to flash

You need the image file you want to flash. Image files have the extension *.nfi. You can't flash other images like *.img files. Don't try it - it won't work.
You need either a DHCP server (usually a router) , a configured bootloader or a serial cable and a terminal program like HyperTerm.
You need a browser, and a network cable. If you connect the Dreambox directly to the PC, you need a cross cable, if you connect trough a switch/hub/router you need a normal (non-cross) cable.

To flash a new image, you can use the second stage loader. It must be present and not broken.

The following steps assume you have a DHCP server. If you don't have one, you have to setup
an IP address in the bootloader's serial setup.

So, how to do it....

Power down the Dreambox, either using "shutdown" and by removing the power connector.Standby is not enough!
Press and hold the power button. Reconnect the power connector into the Dreambox.
The text "*** STOP ***" should appear on the OLED display.
Now you can release the button.
The bootloader has entered the "waiting" mode.
Above the "*** STOP ***" text, there should appear an IP-address (four decimal values with a dot in between). If the text "dhcp" appears instead, the box is still searching for a DHCP server. Wait a bit. If it doesn't change, something is wrong with the connection to the DHCP server.
Check wether network cable is plugged in etc. If it just doesn't work, set an IP using the serial terminal)
Now, open a browser on your PC.
Enter in the address line "http://ip/" (where ip must be replaced with the value in the LCD).
On the bottom of the page, there's a link called "firmware upgrade". Click it.
On the next page, you can select the flash image you want to flash. be sure that it has the ".nfi" extension. You can't flash other images (like .img)!
Click on "FLASH!" and wait. It might take some time to upload and flash the image.
Switch the box off again, and on again. The new image should be flashed and working.