In order do buy a monthly recurring subscription:
 

1. Go to the app welcome screen - the one with three tiles: Meditations, Insights, Karma Dice
    
2. Tap on the menu in the top left corner. 
    
3. Choose "Buy Subscription".
   
   If you are not logged in, we'll ask you to do so :)

In order do buy the annual subscription - check this article.