I have an active subscription taken in the mobile app. Can I purchase an extra year of subscription?

Yes, you can! Just click on the Extend button in your app. 

For Android apps, you will be able to purchase a year of subscription; the unused period of your previous plan will be added to the new one, extending it. 

For iOS apps, your current plan will remain active, and then, once it's over, your new annual plan will start at the discounted rate. 

Note that in the mobile apps, you can take only one annual subscription. If you want to have more, you can cancel the renewal of your in-app plan first and then purchase several years of the plan via the web version.