Unfortunately you will need the previous code or the installer code to be able to do anything with the alarm. Have you contacted the landlord to see if they have a code?
Make sure that code will arm/disarm the alarm (press 2812 AWAY)... If it works ok the come back to me with the model of the alarm which you will have to get off the control box (a box about a foot square in the roof or in a cupboard near the ceiling).
I will then be able to advise how to change the code...
This should be a Solution 880....
You can download a manual here:
To change the existing code:
28121# (STAY and AWAY flash)
_ _ _ _ # (New code then #)
Be sure to notify the landlord you have changed the code and what it is... Failure to do so can make you liable for the expense of resetting the panel if the code is forgotten or lost.