Yes, you can cancel your Myntra order when it is already shipped. While you won't be able to cancel it on the Myntra website or app, you can contact their customer service via phone call or email, request a cancellation, and they would gladly help you.
Another way to cancel your Myntra order when it's shipped is by not accepting the order when the delivery company reaches you. The delivery company will return it to the merchant and you can request a refund.