Only you can cancel your life insurance plan. You can either stop paying your premiums and the policy will lapse and then cancel, or you can contact the insurance company or your agent and request that your policy be canceled.
However, if you develop cancer after your life insurance policy is issued, the insurer cannot cancel your coverage due to the change on your health.
If you have questions about your coverage make sure to send them in writing to your insurance company and get the answers from them in writing.
To learn more, read this article: Does Life Insurance Cover Cancer?