When an Amazon order is cancelled — whether by you in Seller Central, by the buyer within Amazon's cancellation window, or by Amazon itself — the cancellation is picked up by the CedCommerce Amazon Channel app automatically. There's nothing for you to do in the app.
Open Orders in the left navigation. Cancelled orders appear in the All tab with a Cancelled status. You can filter the list by status to see only cancelled orders.
To process a cancellation reliably, initiate a full cancellation from Amazon Seller Central — not a partial one.
Do not initiate a partial cancellation. Partial cancellations aren't supported by Shopify and will fail to sync. A partial cancellation is when an order has multiple items and only some of them are cancelled.