You can manage your billing email inside A2X. To begin, go to your A2X Settings tab and then Billing, then click 'Change' under Payment Method:
Then click on Change Payment
You can change your email in the next form either in PayPal or Credit Card, depending on how you pay. (Your current billing name and email will show. Below is just for demonstration.)
Click Update PayPal Subscription to complete the change. When your next subscription payment goes through, you can either check for the invoice in your email or check A2X Users again and you should see the new email there in the Paid By column.
You've successfully changed your email and it will be reflected on your next monthly billing!