How do I request a refund?
For apps in the App Store the entire purchase process is in the hands of Apple.
We as developers can neither see who purchases our apps nor can we trigger refunds.
If you accidentally baught Wokabulary's Pro Features, you need to request a refund from the App Store support. Apple has more info on how to do in the help article "Request a refund for apps or content that you bought from Apple"