Yes you can cancel a policy after you buy it. A free look period of 15 days is provided to you after buying a policy to understand the terms and conditions. In case there is any objectionable clause you can cancel the policy and get a refund. Stamp duty, expenses on medical check-up and proportionate risk premium (the number of days that the insurance company was at a risk of bearing your health expenses) would be calculated while the premium amount is refunded. Refer the policy termination or policy cancellation section in your policy wording to know the amount that would be refunded.
Note: For the refund to happen there should be no claim during the policy period.