A host can always refund money to a guest if they wish, using the resolution center "send money" option. But it's a really bad idea to refund money until you have actually been paid by Airbnb for the stay or manage to get the dates rebooked, as the guest may ask Airbnb for a refund and receive one- then the host is out the money twice.
It's also a bad idea for a new host to take a 31 day reservation. That could put the guest into a category of tenant as far as your local landlord/tenant laws go, and could cause you possible hassles if the guest refuses to leave when their booking is over. It's much better to take shorter bookings when you are a new host til you learn the ins and outs of the platform and develop skills for properly vetting guests.
However your listing read when a guest booked are the conditions of their booking, whether it's the cancellation policy, house rules, amenities you provide, etc. Changing aspects of your listing info and policies only applies to bookings made after those changes.
@Jenny1247