Typically, your insurance company will send you a renewal offer and you can pay them to renew the policy. Depending on how the billing is set up, they may automatically take the payment out of your account for renewal.
You can also shop for insurance and potentially find a cheaper policy elsewhere. In this scenario, you would tell your current company that you do not want to renew the policy with them.
If you want to learn more about renewing your car insurance, check out this article: How to Renew Car Insurance.