What type of alarm system is it? Not the sticker on the window, take a look at the keypad and the main panel circuit board.
Once you have done that I can tell you how to change the code. Typically you will need the master code to change it so you may need to contact the realtor. There is a link to a Blogger blog on my website (in the archive) where I can easily answer your question.

call another security and tell them about it ,, compare systems,,and then see what you can accomplish