In the Admin Console, find the Subscription Details page by going to the customer's account and clicking on the subscription's name. In the subscription action dropdown menu, you will see the option to "Change Next Bill Date". On the Change Bill Date page, you will see the current date the subscription is supposed to bill and the current hour of today in your user's timezone. Click on the date to select form the calendar the new day you want the subscription to bill. Pick the appropriate hour block for the new bill date.