At the moment, you can only subscribe to the premium version in the Android version of the application.
If you have an Android device, please log into the account you want to get subscribtion on primium version to. Then, in the application tabs, select Subscription management tab, read the terms of premium subscription and if they suit you, click Subscribe. After that, Google Play will be opened where you can subscribe.
After subscribing in Google Play, in the application on the Subscription management tab, you can see that you have a current premium plan.