Accenta 8 keypad not working after power cut?

Hi, I had a power cut yesterday and the keypad no longer works. The key backlights are on, but the power led isnt.

I understand I need a new battery? Could someone give step by step instruction on how to replace it? Is the battery under the keypad or in the main unit? Will the alarm go off if I start to tamper with it? What battery would I need?

Thanks.