Where you manage the subscription depends on where you bought it.
Bought on the website
Open Settings on desktop, go to the Grape Pro tab and choose Manage Subscription. It opens the billing portal in your browser, where you can change the plan, update the card or cancel. Web subscriptions are managed on the web, so the mobile app will point you there too.
Bought on the phone
Subscriptions purchased on iOS or Android are managed through the App Store or Google Play subscription settings, like any other app subscription.
Restore purchases
New phone, reinstalled app, or the paywall showing even though you paid: tap Restore purchases on the Grape Pro screen and your purchase is looked up and reapplied.
Canceling
Canceling stops renewal, and your notes are never held hostage. Everything local stays local, and export and backups work on every plan, see Export your notes.
If something is stuck
Write to support with the platform you subscribed on and the email you used, and a person sorts it out.