When your Premium plan expires, the app will show a message like below. 

Let us show you how to renew your plan.

1. Tap the Setting button (purple)

2. Scroll down and tap "Get Voice4u TTS Pro"

3. Select either Monthly or Annual plan.

You may be asked to type in your Apple ID and password to complete a transaction.

4. Then, you'll see a series of small windows like followings:

Tap "Buy" to go next.

The app says "You've Already Purchased This Subscription" and tap "Buy" to renew. 

(This particular message is sometimes confusing though, this is the requirement from Apple and we must follow their guidelines.)

Finally, you have successfully renewed your Pro plan.

5. (optional) We strongly recommend to create your Voice4u account so that you can use the Pro features on more than one device. You can also recover Pro when the app is accidentally deleted from the device or switch to another iOS devices.

Tap the Sign Up button to create a new account.

Type email address and password. Then, tap "Sign Up".