You will need to wait for the related purchase invoice to arrive from Apple via email - this usually arriving with 24 hours.
In many countries/regions, App purchases are final. However, within the invoice email will be a “Report A Problem” link - where you may request that an App purchase is cancelled. Apple, at their absolute discretion, may cancel the purchase and make a refund to the registered Payment Method that was used to purchase the App.
You should note that discretionary cancellation/refund is not a mechanism for testing software - and returning if not to your liking.
More information about requesting refund for unintended App purchases can be found here:
Request a refund for apps or content that you bought from Apple - Apple Support