@Judy239 ask the host to change the booking and you'll get asked to confirm it. If the host does not know how to change the booking, tell them to go to their reservations and click "change" on your reservation.
Thanks Jeff. The host has told me, quote - Airbnb deals with cancellations and dates adjustments. So once you contact them they will be able to help you with any dates adjustments...
I did copy and paste to him on the message board the instructions from Airbnb that I had followed, and how he should respond, but he hasn't replied.