Hi @Ashley1808 🌿
I would write to them : Dear guests - I so sorry that you had to leave early. When you leave you will need to cancel your reservation and you will then automatically get refunded according to the agreed to cancellation policy. It all happens automatically as the Airbnb system is set up to refund / per the cancellation policy. When you cancel - the calendar will reopen so new guests will be able to book.
If your guests won’t get a refund - because they are not entitled to one per the cancellation policy I would write : And should any of the days rebook I will gladly refund you those days. I wish you well and hope to see you another time.