Ademco Vista 6139 keypad.
As suggested by others, this will be wired back to a bigger box somewhere in the building. Any monitoring devices like door contacts will also be wired back there. Look for any references to the installer - it may still be under a monitoring & maintenance contract.
If no contract or out of contract, look up some manuals for this series and try getting into engineer mode. I think the default with these is 4110 800 iirc. If you can get engineer, you can put in new user codes for... using your system. It's likely the engineer code has been changed though.
So break in. Most boxes are equipped with a tamper switch, so it is likely to scream if you remove the cover. If you decide to do so, switch off the mains supply first. Then remove the cover and immediately disconnect the battery to kill it.
From here, ID the unit and find the installer or engineer manual. The user manual is useless at this stage. Familiarise yourself with it.
When you are ready to play, pop the battery back on, restore power and then go immediately back to the keypad to press * and # together. This should reset the engineer code to the above, but retain the rest of the current programming.
These are old systems and a lot of this is from memory, so YMMV.
Alternatively, get the codes off the previous owner or replace it with something new. This one could be over 20 years old; vistas started in the 90s.