Well done, getting there!
Hint - easier both for you and us - using PuTTY for the serial comms, if you click for the top left menu, you can use 'Copy all to clipboard' and paste text into Notepad or similar to save/edit /copy etc.
As you have the required firmware - here is what you should be able to do:
Know what IP address the PC is using. Let's say it's 192.168.1.xx
Install a 'normal'
tftp server on the PC. The Jounin one, 32 bit version, works quite well :
TFTP server
Place the digicap.dav firmware file in the same folder.
With the NVR powered off, start PuTTY.
Keep Control-U pressed on the keyboard so it auto-repeats.
Power on the NVR.
This should interrupt the bootloader and give you probably an HKVS # prompt.
At the prompt, use the command
printenv
to show current environment variables.
You'll need to (temporarily) change a couple to save having to change the PC IP address.
For the PC tftp server IP address :
setenv serverip 192.168.1.xx
For the NVR IP address, pick one in the same range, for example :
setenv ipaddr 192.168.1.200
These are not being saved, so won't make a permanent change.
Now use the command
update
which should download the firmware file and verify it and carry out the update.
Good luck!
Feel free to copy serial console text here, pasting it into the Code tags (the '+' in the Edit menu) if there are any problems.