There are two ways to change the way a subscription gets recharged or billed:-
1) Make changes to the billing period that the subscription is attached to
Go to the customer dashboard and click on "settings" in the subscription block
A pop up will appear, click on "Add Override" under the frequency that needs it's billing date changed.
select the new billing day from the options under the billing day column and click save
Note:- this change will affect all subscriptions with that frequency
More information on Billing Periods can be found here Customer Properties: Billing Periods
2) Alternatively, the billing date of a single subscription can be changed
Go to the customer dashboard->subscriptions
Click on edit for the subscription that needs it's billing date updated
Click "modify" for the next billing field
Click "modify" under the invoice day field
click on the day of the month, you wish to change the subscription to
or the day and month for a yearly subscription
Note: The system will change the billing day to the next instance of the date entered. E.g. If a user is trying to change the next billing day from December 6th to December 14th on November 8th, the system will change the next billing day to November 14th. In order to prevent this from happening, wait until on or after November 14th to make the change.