N.B. The screenshots and descriptions are for the iPhone/iPad version.
Subscriptions are managed through the Stable Manager web app at app.stables.cloud. Log in, go to your Stable Settings, and select the Subscription option to view available plans and upgrade.
Subscriptions cannot be purchased through the iOS or Android apps. This is because Apple and Google charge a significant commission on in-app purchases — going via the web means more of your money goes towards the service.
Once you have subscribed on the web, your upgraded plan is available immediately across all your devices.