You are free to cancel your free trial to Capture One for iPad and resulting subscription at any time. See how to do so here.
If you wish to request a refund for whatever reason, you will need to contact Apple Support, as all of the transactions related to Capture One for iPad are handled by the App Store.
See how to request a refund from Apple here: https://support.apple.com/en-gb/HT204084
When you cancel your subscription, you will still be able to access the iPad application until the end of the current billing period.