Optima mx6 how to reset code?

Tenant left after 6 years. Electrician then fitted hard wired smoke detectors. Mains power was isolated to do this. When mains power came back on the Alarm went off and the electrician didn't know the code so he just pulled the fuse above the control panel. No one knows the old code except the tenant who has now left!
How do we go about putting the fuse back and resetting a new code?
Any Advice gratefully acknowledged.
Cheers