How do I cancel/modify the remainder of my booking?

I have a client book 3 drop ins, once a day for 3 days. I completed the first check in this morning and the owner realized he double booked sitters and no longer needs the remaining 2 drop in and needs to be refunded for them.

Is this considered a cancelation that the owner needs to complete? Or is there a way to modify the current booking to remove the remaining drop in's and if so, how is this done?

Thanks!