If there are any or all Reservations/Group Rooms that need be cancelled with a Group that is synced to Mews:
Click the Reservations tab in the Booking Summary
Check the boxes for all reservations needing to be cancelled, then select the Batch Action for Cancel
Click Apply - you will be given an option to charge a cancellation fee and add notes
Click Cancel Reservations
You will then need to change the Group Status to Lost to cancel it in Mews:
Navigate to the Group Details section of the Booking Summary
Click Edit
Change the Status to Lost
Click Save
Delete the Group
You can now re-add a Group to the Booking with released rooms, or resell them to another Booking/Group
If the whole Booking is cancelled/Lost, you can also change the Booking Status to Lost.