A medical insurance policy can be cancelled after it is bought complying with the terms and conditions in the policy. Usually a free-look period of 15 days is provided after purchasing a policy to understand the coverages and terms & conditions. In case there is any clause which dissuades you from continuing the policy, you can cancel the policy and get a refund. Stamp duty, cost of medical check-up and proportionate risk premium (for the days the insurer was at a risk of bearing your medical expenses) would be computed while the premium amount is refunded. The refund formula may defer from plan to plan and should refer the termination or policy cancellation clause in your policy wording to know the refund amount.
Note: To get a refund, there should not be any claim during the policy period.