If you purchased StudySmarter Premium through the iOS App Store (explained here) or Google Play (described here), you can manage your subscription from those stores.
In case you purchased StudySmarter Premium in our Web App, you can manage your subscription in your profile on the StudySmarter Web App or you can manage it in your PayPal account (explained here) directly.
For any clarifications, please email support@studysmarter.de. To help you as quickly as possible, please send us the email address of your StudySmarter account and the channel (App Store / Play Store / Web App) through which you purchased StudySmarter Premium.