Hello @Teresa2618
Here is the Airbnb link of how to change a guest reservation from your end - one also exists for the guest but in view of the urgency, I would suggest you do it from your side. The guest will have to accept the change and any price difference.
https://www.airbnb.co.uk/help/article/50
Let him know by message (use the Airbnb message stream) that he will be getting this request that he must approve immediately!
How did they get their dates so wrong, do you know??
Let us know how this turns out,
Good luck
Joëlle