The MTOM is a parameter that is part of the Type Design Standard of an aircraft. A change to the MTOM is a design change (modification) and must be approved by the relevant regulatory authority through the auspices of the aircraft manufacturer, or another suitably approved design organisation. Such a modification could be generic to a type, or aircraft specific. The embodiment of an approved modification is the only means of permanently changing the declared MTOM for an aircraft.
Operators who embody modifications which reduce the maximum declared take off mass must advise the CAA Applications and Certifications Department to ensure that the aircraft register database includes the correct data. We will also need to know the provenance of the relevant modification's approval.