If you downloaded Outlier through the Apple App Store, you can only manage your subscription directly through Apple.
Cancelling Your Outlier Subscription with Apple: https://support.apple.com/en-us/HT202039.
Requesting a Refund: https://support.apple.com/en-us/HT204084
We really wish that we could handle on our end, but sadly, Apple prevents us from handling subscriptions and refunds.