To flash a DM800 read below.
Make sure the box is networked then switch the box on and then immediately hold the power button in.
It should then display on the front something like the following
****stop****
192.168.1.5
If you go to your pc ( has to be on the same network) open up firefox, or which ever browser you use and type in the ip address that is displayed on your DM800 you should then be greeted with the loader page.
You can then use this to send a new image to your box.
Make sure you find out which Sim & bootloader your box uses as if its from payserver its unlikely to be a genuine box.
You will then have to find an image which is correct for that bootloader and SIM. If you install the wrong image you will get a "No CA" error and the box will not boot up.
Sometimes the information will show on the box when it boots up.
My old clone used to displayon the front of it something like the following when it first booted
SIM 2.01 #82
Translated, SIM 2.01 Bootloader 82
So for my old box you would need to find an image for a SIM 2.01 box with bootloader 82. If it doesn't display anything you will need to open the box and have a look. It usually says which sim it is on top of it.
You should see something like the images attached.
Nano