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.
If you purchased StudySmarter Premium directly on the platform, you can manage your subscription in your profile on the StudySmarter Web App:
If you have any questions, please email support@studysmarter.de at any time. 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.