cancelation of reservation less than 24 hours before arrival and giving back guest money to the guest

I would like to cancel booking  less than 24 hours before arrival and give back guest money to the guest, please help its urgent. My guest is open to do cancellation from his side but I don't know if its possible, from my side I see an information that it is possible 48 hours before arrival via some Booking contact/ help but didn't find any telephone number to talk to anyone to help me to resolve this issue. My calendars from Airnb and booking didn't connect properly and my guest booked the date which is already booked on. Airnb. I already informed him and apologized, although we both would like to cancel that. Dear booking.com and partners, please help to a new host here, thank you