Where can I edit the customer's subscription?You can edit a specific subscription by going to your App settings -> Subscriptions -> click on a subscription -> Edit subscription. This functionality allows you to edit every part of the subscription, including product prices.
You can also edit a subscription through the customer portal (if you have turned on the feature which allows your customers to edit their info). You can turn on the features which allow your customers to edit their subscriptions in App settings -> General settings. You can access the customer portal if you go to your App settings -> Subscriptions -> click on a subscription -> click on the "Access customer portal" link.