How do I cancel a subscription? It only gives an option to resubscribe.

If there is no cancel option and if it displays a resubscribe option, both of those things indicate that it is already cancelled. See the following from How to cancel a subscription from Apple - Apple Support

If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew. 

and

If you cancel a paid subscription, you can keep using the subscription until the next billing date.