You can cancel a canvas print order within 24 hours of placing the order.

To cancel an order:

1. Go to your account settings, by hovering over your avatar in the top, right-hand corner of the screen and selecting Account from the drop-down menu that appears. 
2. Click on the Purchases tab in the menu on the left. Alternatively go to http://500px.com/settings/purchases 
3. If your purchase is still available to cancel, simply click Cancel Order to stop your order being processed. You can also update your shipping address here.