Users can view the status of all of their subscriptions and cancel them if necessary from the My Apps screen in the Play Store app. Currently, the In-app Billing API does not provide support for programatically canceling subscriptions from inside the purchasing app.
When the user cancels a subscription, Google Play does not offer a refund for the current billing cycle. Instead, it allows the user to have access to the cancelled subscription until the end of the current billing cycle, at which time it terminates the subscription. For example, if a user purchases a monthly subscription and cancels it on the 15th day of the cycle, Google Play will consider the subscription valid until the end of the 30th day (or other day, depending on the month).