how do i cancel apple care

The article at the top of the page provides the info you need to start the process. Cancel or change the payment method for your AppleCare plan - Apple Support

I usually call Apple support and I ask to cancel it and have a refund of unused months, if you pay monthly you can login your apple care account and cancel it

Depends on how you got your monthly subscription. If purchased through a retailer (like Best Buy for example) or your carrier you would contact them to cancel.

Interesting, best buy it directly from Apple on the iphone then, the price is the same so why buy it from someone else?