Thanks for contacting TextNow Support! You should be able to cancel it simply by trying the following:
1. Open up the TextNow app on your mobile device
2. Click on the top left parallel bars to open up the menu- click on the "Premium" option
3. On that page, there should be a button to "Manage Subscription" or it may be labelled as "Check Subscription Status" based on your mobile device
4. From there, you should be able to cancel the auto-renewal subscription!
Let us know if you run into any issues :)