If you started the trial period, you can easily upgrade to the Lifetime subscription.
First, you need to cancel your current trial.
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 the button with your subscription at the top of the screen, and purchase Lifetime.
Go to Play Store app → left menu → Subscriptions → there choose App in the Air → cancel subscription.
Then go to App in the Air → Profile → 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.