If you purchased Vaia Premium through the iOS App Store (explained here) or Google Play (described here), you can manage your subscription from those stores.
If you purchased Vaia Premium directly on the platform, you can manage your subscription in your profile on the Vaia Web App:
If you have any questions, please email support@hellovaia.com at any time. To help you as quickly as possible, please send us the email address of your Vaia account and the channel (App Store / Play Store / Web App) through which you purchased Vaia Premium.