Hi @Lucy1119
It sounds like the host's calendar is blocked for December, but hopefully they would've mentioned it to you if there were bookings in place. So it's likely that they've set an availability window of 3 or 6 months, which is keeping December blocked for now.
There's nothing you can do if this is the case - the host will first have to extend the availability window on their calendar to include December. Or alternatively, there's an option that they can tick to allow booking requests beyond their availability window.
Once the calendar is open and bookable for December, you can click on your reservation and select "Change Reservation" near the bottom of the booking to request a date change. The host will have to approve the change.
If the host doesn't manage to extend the availability window, or if I've misdiagnosed the issue, they can always ask here in the Community Center and there will be plenty of people to assist.