@Terri-and-Martin0
As reservation is already confirmed, you can not change it. Also the first installment is already paid to Airbnb by the guest. Long term stays can be cancelled within 48 hours (if not starting within 28 days) with full refund, but as a guest i would not take that risk.
If you change the reservation ("change option") and amend the price, maybe it will recalculate installments, but as a host i would not take the risk that something goes wrong.