If you started the trial period, you can easily upgrade to the Lifetime subscription.

First, you need to cancel your current trial.

iOS: Go to App in the Air > Profile > Settings > Manage Subscriptions > there you will be redirected to the iTunes app. Select App in the Air, and tap Cancel. 

Then go to App in the Air Settings > tap on button with your subscription at the top of the screen, and purchase Lifetime. 

Android: Go to Play Store app > left menu > Subscriptions > there choose App in the Air > cancel subscription. 

Then go to App in the Air > left menu > Settings > Manage Subscriptions -> purchase Lifetime.

If you already purchased the annual subscription, and want to change it to Lifetime, contact our Support Team through the app's settings feedback option.