You may need to edit your customer's next payment date associated with a subscription for a variety of reasons. This can be done on the detailed customer view.
NOTE: The script that generates invoices is run twice per day - once at 6:00 AM EST and once at 4:00 PM EST. If you are adjusting the next payment date/time in order to generate an invoice, please be aware of these times as it may take several hours for the invoice to generate depending on the time selected in the next payment date.
Navigate the customer you want to change their subscription invoice date for under People > People and click Edit. Then click the Access tab and the three dot menu on their active subscription plan. You can change their invoice date on the next page.