I dont use one on mine to be honest, although i am using an old Egami image as it does what i need it to do, so i cant see the point in changing it.
To change the firmware all you need to do is download one, unzip the file and copy it to the root directory on a usb pendrive. Make sure its a decent Pendrive like a Kingston or a Sandisk one as Venton receivers are funny buggers when it comes to USB drives.
You should have on the pendrive a folder called "Unibox" Inside that folder will be another folder called "HDE"
Inside that folder will be several files..
Imageversion, kernel.bin, noforce, rootfs.bin and splash.bin
Make sure you have downloaded the correct image for the box and not one for a HD1 or HD2 receiver. Flash the wrong image and you have a scrap box.
Once you have confirmed this, switch off the receiver and insert the usb drive into the top usb port on the rear. Switch on receiver.
It will say "booting" and will the state there is an image on the usb stick. It will say "press power to flash".
Press power button on front of receiver and it will say "flashing" on the receiver display. Do not power it off. When complete it will say "success" on the display and it will reboot into the new image. Follow the instructions to set up the image. You can remove the usb stick at this point.
Just be aware if you are going to be using an emu that not all images allow yo to download them, although most do.