You can change a subscription via the Admin Console or the API. Recurly does not support subscription changes by your customer through the Hosted Pages.
In the Admin Console, find the Subscription Details page by going to the customer's account and clicking on the subscription's name. At the top right of the page you will see a "Subscription Actions" dropdown where you can find the option to "Edit Subscription". On the Edit Subscription page, make your changes and click "Save Changes".