A lot of how this should be done depends on how seriously you take the security of your home.
I could give you the advice you need as could pretty much every other person on here, but there are several issues with that.
If you do it incorrectly you could harm yourself, your home or even be very cross with the person that advised you.
It seems very simple changing a battery, and it is, if everything else is ok. But the chances are that if a proper alarm engineer looked at the system he ould do a very good job of it. No offence but when a customer says the alarm was installed by an electrician it sounds bad, like as in "my alarm engineer did my electrical work in my house" .
The engineer code should not be the one in the manual/as standard.
this would make it too simple for anyone in your home to alter the systems programming.