Manufacturers often leave backdoor passwords. Try these in order:

Download the correct firmware from the manufacturer’s site. Use the DVR’s local update menu (if you can guess a low-privilege user) or a USB update during boot.

Do not use any “universal password generator” tools found on shady forums – they are often malware. Do not pay for “remote password reset” services claiming to hack your DVR; they will likely just use the free methods above. If you are locked out of a DVR from a defunct company, your only real options are hardware reset (jumper/battery) or replacement.