No part of a booking is refundable, moveable or exchangeable once it has been confirmed at checkout.
Firstly, as you are only wanting to change part of the booking, we would advise you to try and identify other friends or family members who can join your party instead of those who are unable to attend. If you successfully identify other attenders then you do not need to do anything else, but pass your tickets onto the attenders. Please follow the advise here on how to transfer your tickets.
If you are unable to identify friends or family able to use the tickets allocated to those who will not be attending, then it is necessary to prioritise: