If you subscribed to RINGR via Google play (rather than via the iTunes Appstore or via credit card at https://account.ringr.com), you can manage your subscription either via the RINGR Mobile app (on Android) by using the menu to open the subscription panel:
Or via the RINGR account site (in a browser). Head on over to https://account.ringr.com/#account.
In either app you'll find options to upgrade or cancel your current subscription. Once you've cancelled you can re-subscribe to any of the available RINGR Basic or Premium plans. Don't worry, you won't be billed until your current subscription expires, or if changing plans we automatically 'pro-rate' the charges deducting any remaining money for your current subscription from your bill for the first month of your new plan.