How do I cancel a pending

You cannot cancel it once it's tried to be charged. You must let it complete, and then request a refund once its done.

If this is a one time payment for an App or a movie or TV show, see here once it successfully completes: link-> Request a refund for apps or content that you bought from Apple - Apple Support

If its recurring subscription, cancel it first: link-> If you want to cancel a subscription from Apple - Apple Support

Then contact Apple support for a refund once its completed successfully: link-> Contact Apple Support