For future bookings, you can change your policy in your manager portal under Site info tab / Payments.
For past bookings, the cancellation policy in force at time of booking is shown in the email confirmation or in your Bookings tab. You cannot tighten this cancellation policy retrospectively, but can offer to make it more favourable. For example, if a customer decides to cancel, you may want to offer a postponement if your normal policy is to retain the balance.