Cancel at least 24 hours before your renewal date. Apple's terms say you're charged no more than 24 hours before the next period starts, so cancelling inside that last day can be too late to stop the charge.
Cancel your Soco subscription from the app
Apple handles all Soco subscription billing, so the app hands you to your Apple settings to finish:
Open the Soco app.
Tap the account icon in the top right.
Under Help, tap Manage subscription. Your Apple subscription settings open.
Follow Apple's prompts to cancel Soco.
Apple's subscription screen then shows Soco as expiring on a date instead of renewing. That means it's cancelled.
Cancel your Soco subscription if you can't open the app
You can cancel your Soco subscription from your Apple Account without the Soco app. Apple's own guide covers every device: If you want to cancel a subscription from Apple. The confirmation is the same: Soco shows as expiring on a date instead of renewing.
What happens after you cancel
After you cancel your Soco subscription you keep Pro until the end of the period you've already paid for. You aren't charged again as long as you cancel at least 24 hours before your renewal date. Apple's own terms say you are charged no more than 24 hours before the next period starts, so a cancellation inside that last day can land after the payment has gone through: Apple Media Services Terms and Conditions, under Subscriptions. Deleting the Soco app or deleting your Soco account does not cancel the subscription; only cancelling in your Apple subscription settings does.
Next steps
Your next question is probably one of these: