Denon D-850 (UDRA850) changing AM step

[soundrepairs]

Hi,
does anyone have any idea how to do this. Unfortunately my customer has lost the owners handbook and we wish to change the AM step from 10KHz to 9KHz.
There is no switch on the back of the unit and it's too old to be done through a firmware upgrade which leaves the likely possibility that pressing a combination of 2 or 3 front panel buttons toggles the AM step between 9 & 10KHz.
Does anyone know the procedure please?
Thanks