How can I modify a customer’s next scheduled renewal date?

The subscriptions renewal date for an individual active subscription can be changed at anytime from the subscription's details page. From the customer's account, you'll want to click on Options near the subscription summary window then click on View Subscription. On the next page depending on if there is a trial period or not then you will see Change Next Bill Date or Change Trial End Date on the left-hand side of the page under the subscription summary window. You'll want to click on that button respectively in order to change the next billing date.

Please note that by changing the renewal date, you are changing the day for when the subscription will renew moving forward.