Amending existing booking

We have a client who has booked 4 pax for 20 nights.
The booking has been confirmed and paid by cc via Booking.com.

They have not checked in yet.

The client has contacted us now to request a change of dates, arriving one day later, reducing their stay by one night.
They will also only be 4 pax for the first 8 nights, and then only 2 for the last 11 nights.

Our refund policy is 50% non-refundable.

How do we go about changing the booking accordingly?
Who makes the change, and if the client must request it, who does he contact, because he is not managing on the booking.com site.
If we decide to waiver the 50% penalty, how can we do that, and will booking.com allow the waiver, or will they still charge us commission on the full original booking cost?

None of the phone numbers for booking.com in Cape Town are currently operational, any advice would be greatly appreciated.