Yes you can cancel your car insurance immediately at any time you want too.
It's best however though to stay with your current car insurance company for at least 6 months to a year before canceling a car insurance policy.
Because canceling your car insurance policy too soon and too often can look bad to other car insurance companies and they may not want to insure you or your car if they think you're just gonna cancel immediately or soon after.
But if you found a much cheaper car insurance company and a better car insurance company after you signed up for a current car insurance policy then yes you can contact your car insurance company at any time and let them know that you want to cancel.
Sometimes letting the car insurance company know that you plan to cancel your car insurance will get them to give you a lower car insurance rate.
So it's worth a try.