The discount code should be applied during checkout but before payment. This is important because once the payment is made, you cannot undo it without technical support. So remember, always verify that your discount code is applied and showing you the reduced amount in the final price. The following image shows when and where the code needs to be applied:

Yes, you can use your discount code whenever you want, just make sure to use it before its expiration day.

At the moment, it's not posible from "my account" page. Nevertheless, you can send and email to us describing the changes you want to make and we do it for you. We are working to offer you a better experience, so this option will be availbale soon.