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It is better for you to request the guest to cancel.  At this late notice try and call them.  Explain reason and do some work beforehand so you can recommend somewhere close by.  If you ask booking.com to do this they will charge you a relocation fee.

If you need to contact booking.com help team using Inbox icon on Extranet or give them a call see link for local phone numbers. 

https://partner.booking.com/en-us/help/where-you-can-reach-us