You can cancel your car insurance every year with one month's notice. The date the insurance company receives the cancellation is decisive.
Most insurance policies expire at the end of the calendar year. In this case, the insurer needs to receive your written cancellation notice by 30th November of the current year at the latest.
In Germany, in some cases you have special cancellation rights. For instance, if your insurer increases your premium without offering a better policy, you can usually prematurely cancel the contract. In this case, your cancellation must be received by your insurance company no later than one month after you received the contribution increase.
You also have special cancellation rights in the event that you are dissatisfied with how your insurance has dealt with a claim.
Furthermore, in the case that your car insurance is no longer necessary, you can terminate it immediately, e. g. if you sold or scrapped your car. In this case, notify your insurer in writing as soon as possible.