How to fix bell box back on wall?

Hi, we have an Optima Compact alarm, the external bell box for which has recently slipped. Have tried rehanging it, it worked for a while but recent windy weather has moved it again. Need to remove cover from bell box but understand I need 'engineer code' - originall installer moved from area. Can I do this myself, if so, how do I find out engineer code or is it the case of getting someone out? If I do have the code, sounds a silly question, but what exactly do I do with it e.g. do I enter it on main keypad in house? Would be grateful for advice. Thanks.
Andrew.