Redirekt Pro is a monthly subscription, so it can be cancelled at any point during the current month until 24 hours before the billing date.
Subscriptions made via the App Store, such as Redirekt Pro, must be managed from your iTunes account. You can find further information on how to cancel a subscription on the official Apple Support portal.
Please note that if you cancel less than 24 hours before the billing date, Apple may still bill you for the next month. In this case, you'll continue to have access to Redirekt Pro for the entire period you paid for.
If you received your subscription as a gift or at a discounted rate from outside of the App Store, your subscription is automatically cancelled at the end of the promotion period. We do not collect payment information, so you won't be billed for anything that happens outside of the App Store.