How do I cancel my Audible membership on my iPhone?

No one can cancel your membership here. This is a user to user only forum and no one here is associated with Apple at all. If you want to cancel your subscription see how to do that here --> If you want to cancel a subscription from Apple - Apple Support

Cancel it through Audibles website

You can find instructions here:

https://help.audible.com/s/article/cancel-membership?language=en_US