If you purchased iRacing directly from the iRacing website (www.iracing.com), with a credit card or Paypal account, you can cancel your membership by logging into http://members.iracing.com and then click Account...My Account.
On the My Account Home page, scroll down to the Billing Options section, click the "Click Here to turn off auto-renew" link. If it now says "Click Here to turn on auto-renew" - you have successfully turned off auto renew.
This will turn off the auto renew feature and your credit card or Paypal account will not be charged the next time your subscription expires.
If you purchased iRacing from Steam, you will need to contact Steam Support for further assistance.
If you purchased iRacing using PaymentWall as a payment source, you will need to turn off the the auto renew with PaymentWall. See https://www.paymentwall.com/en/faq for more information.