How to cancel and refund my HBO max subscription?

See these two links:

How to cancel a subscription from Apple - Apple Support

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

Note, your request for a refund is not guaranteed as Apple's policy is all sales final. Still, it doesn't hurt to ask for it, just don't be surprised if you aren't granted one. Cancelling the subscription will ensure you aren't billed going forward.

Then cancel the subscription so you don't get charged again.

https://support.apple.com/en-us/HT202039

Failing to cancel the subscription before the next billing period begins is not a legitimate reason for a refund.