First, go to Your Orders page, then locate the order that you want to cancel. Click “View and edit order” as shown below:
Afterwards, click on “Cancel items”:
On the next page, make sure the items you want to cancel are selected (denoted by the check sign on the right), then click on “Cancel selected items in this order”:
You may also refer to the guide on order cancellations from Amazon: https://www.amazon.com/gp/help/customer/display.html?nodeId=GSL37WQTJZUYA9QE