How can I refund my Strava subscription?

Follow the steps mentioned in this article to request a refund from Apple:

Request a refund for apps or content that you bought from Apple - Apple Support

But not all In-App purchases can be refunded by Apple. In that case you should contact the developer of your app about that, as explained in this article:

How to contact an app developer - Apple Support

If the purchase is not listed in the list of your recent purchases, you will have to contact the Strava Support about that.