Ok, did they actually reserve it as in booking confirmed or was it just a pre-approval? If a pre-approval, then as @Sandra mentioned it expires in 24 hours; pre-approvals cannot be removed but in view of the expiration it doesn't matter, the guest can just send you another request to book the new dates.
However if it was a reservation, then you can alter the dates via the resolution centre; ask them to send you a request to change dates but do not cancel otherwise you will get penalised. Also if they cancel, they will lose the service fee so altering seems the best option, especially as same number of days and only one night apart.
BW, Ama