How do I change my monthly subscription pay date.

The only way to change the payment date is by cancelling the subscription and starting it on another day. It will then reoccur every month on that day.
You could also wait for your subscription to end and then buy it on the day you would like to use as your monthly payment-date/day. Keep in mind that you will have to turn off auto-renew to do that.

Thank you for contacting support. Auto-Renew can be cancelled directly from your App Store or PlayStore app. Please find the instructions for your store below:

Google PlayStore:
View, change, or cancel Google PlayStore subscription

iOS App Store:
View, change, or cancel your App Store subscription


Thank you for replying. I understand.