How do I cancel a subscription during the free trial period?

You cannot cancel individual years like that. You cancel the subscription itself in its entirety not individual years.This includes any free trial period it may have.

If you've already canceled it, then you just need to wait until it expires. At that point it will not renew. There's nothing more you need to do.

Cancelling early however, does not prevent you from continuing to use the trial until it expires.

If you need to have it expire immediately for any reason, then you'll need to contact iTunes Support: https://www.apple.com/emea/support/itunes/contact.html

The same way you cancel a regular subscription. Follow the steps here: See or cancel your subscriptions - Apple Support

You can only cancel the subscriptions you took out. It can't cancel next year's subscription unless you took out a subscription for next year.