If you wish to stop using Capture One for iPad, you can at any time cancel your subscription and future payments.
When you cancel your subscription, you will still be able to access the application until the end of the current billing period.
Simply navigate to Settings > Apple ID (your name, along with your display picture at the top of the Settings menu) > Subscriptions > Tap on Capture One > Cancel Subscription
See also this guide from Apple: https://support.apple.com/en-gb/HT202039
You can also cancel your 7-day free trial at any time by following the same steps. However, you must do this at least 24 hours before the trial ends.
If you are in need of a refund for whatever reason, then see how to contact Apple and raise the issue with them.