You can subscribe a Prime plan from the web or from the mobile app. 

We recommend you to subscribe from the web because it's easier to manage your subscription. For example, you can easily upgrade to a new plan or cancel your current plan; you can easily change the payment card to a different one. For iPhone in-app purchase, you need to go to your iTunes account to cancel or change your subscription; you need to change your iTunes linked card in order to change the payment card . Also Prime Business Plan is only available from the web. Coupon code can only be used from the web. 

To subscribe from the web, please logon to our web version using the same Google account you are using on your mobile app and then click Subscription menu on the left. Here is how the web subscription page looks like:  

To subscribe from the app, click the Upgrade button on Receipts screen or Ad Watch screen, then follow the on-screen instructions. 

You can also click the Subscription menu on the More screen to check your current plan or upgrade to a Pro plan.