Please follow the following instructions from the appropriate marketplace from where you purchased the subscription:
Google Play: https://support.google.com/googleplay/answer/7018481
Apple App Store: https://support.apple.com/en-us/HT202039
If you subscribed with Fastspring, please contact us with the order number or the email address used to make the subscription. Fastspring also sends you an email with a link to manage your subscription.