We send email notification days prior to expiration. But you don't need to wait for it until it expires. Here is the instruction on how to renew prior to expiration.
1.) Go to App Settings > Subscription
2.) In Subscription go to Purchase.
3.) Click 'Place my order' button
4.) And Extend Subscription prompt will appear. Click Extend.
5.) Click 'Confirm' and you're done.
6. ) Go back to Settings > Account and click the refresh button in the upper right and it will be updated immediately.