You can change the address or the payment method for a pending order by clicking on the individual order in your Order History.
You can make changes to multiple addresses or payment methods on multiple pending orders by clicking on the Your Account link and selecting the appropriate menu item (e.g. My Addresses)
To make other changes to an order, please contact customer service as soon as possible at customer.service@paizo.com or 425-215-1442 (Monday–Friday, 10 AM–4 PM Pacific Time). While we will try our best to cancel an order, we may not be able to stop or make changes to an order once the order has been submitted.