Quickly change the Billing Day of the Month for your customers. Go to the customer's profile>>Membership tab>>click desired membership>> select to Edit membership.
At the bottom of the membership edit form you will have the option to change the billing day of the month, as seen below:
You may want to charge your customer extra or less on their next payment depending on which date you changed the billing date to i.e. if the customer paid on 1st Sept and you move the bill date to 15th with the next bill due 15th Sept, the customer is likely owed something.
To do this, head to Upcoming Payments from the page you land on and click on the next upcoming payment.
You will then be able to edit the payment amount for that specific payment only: