If you have purchased the Musicnotes Pro - Premium - Annual or Musicnotes Pro - Plus - Annual membership subscription from within the Musicnotes Sheet Music Player for Android app, you will need to manage your subscription through the Google Play account with which you had purchased the subscription. To change plans, the subscriber will first need to cancel their existing subscription, and then purchase the other subscription.
Please note: You will be unable to manage the subscription purchased from within the Musicnotes Player for Android app - upgrade, downgrade, cancel, etc. - from the Musicnotes website, or be able to change Musicnotes Pro subscriptions on the Musicnotes website.
Please refer to these instructions from the Google Play support site: https://support.google.com/googleplay/answer/7018481?co=GENIE.Platform%3DAndroid&hl=en