I would like to cancel my warranty service for my iPhone

Warranty is free with every device, there is nothing to cancel about it. !ts one year coverage for manufacturing defects.

If you mean the Apple Care+ extension plan, see here: Cancel or change the payment method for your AppleCare plan - Apple Support

Hi serifepotuk,

If you are referring to the One Year Limited Warranty, this is something that comes free with every purchase of an Apple Product and can not be cancelled as it is a manufacture warranty. If you are referring to AppleCare+ you can contact Apple Support at 1-800-275-2273 to cancel your AppleCare+ Support Agreement.

Thanks,

Mr. helper

Follow the steps in the link provided. Link->Cancel or change the payment method for your AppleCare plan - Apple Support