@Jason-And-Briana0
It is not possible to "change" a reservation to another booker.
Solution:
The current guest has to shorten the reservation, the new guest has to book for remaining nights
Please note:
The current guest could have added the other guest (must have Airbnb account) to the reservation before check-in . If you and Airbnb agree (ask..) the current guest can leave halfway and the other guest can stay until the end. But it is too late now for this solution