Changing my payment date
Hi I would like to change my prime subscription payment DATE to the 21st of every month. How do I do this?
If you're talking about a subscription you would have to cancel the subscription, wait for it to end, then start it again on the desired date.
View, change, or cancel your subscriptions - Click on the link to learn "How to cancel a subscription from Apple." --> https://support.apple.com/HT202039